Here is how to back up the registry key before you make changes incase you mess up.

Click Start, then click the Run command.

Type Regedit in the Open text box and click OK.
As an example, go to the following Registry key:

Backing Up and Restoring the Windows Registry

It is always wise to back up your registry before making any changes to it.  This applies, especially, to the novice or to anyone who isn't certain of the effects the changes being made will have on the operating system.  One little mistake with reg editing can take down Windows to the point of not being able to boot.  Backing up the registry before editing makes any errors you make repairable and thus registry editing becomes safe, rather than risky.

There are several ways to backup the registry. However in this article, I will focus on the auto and manual methods built into Windows.

Windows 95
 
Auto Backup

The Windows 95 registry is made up of two files, SYSTEM.DAT and USER.DAT , located in your Windows folder. These registry files are backed up automatically after every successful start of Windows. The backed up files are named SYSTEM.DA0 and USER.DA0 and are also located in your Windows directory.  Should Windows fail to start due to a registry error, you will be prompted to restart your computer to restore your registry. During the reboot, the backed up da0 files are used to restore your registry to the previous state it was before the error. This method, however, is by no means fail proof. 

It is quite common to end up with corrupted dat and da0 files. This can easily happen when restarting after a bad regedit.  Therefore, it is wise to manually back up the registry before making any system changes.

Windows 95 also contains an additional system.dat backup file called System.1st located in your root drive (usually c:) This represents your registry as it was when you first installed Windows. If your registry becomes so corrupted that you can not recover it, you can always try replacing c:\windows\system.dat with c:\system.1st and  then renaming system1st, system.dat. In my experience, however, this rarely works and it's usually easier to just do a reinstall. The reason is that System1st is very bare bones; it doesn't include all the many programs you may have installed after installing Windows.

Manual Backup 

In Windows 95, manually backing up the Registry is a simple matter of copying system.dat and user.dat and saving them to a safe location (personally, I back up my registry files to a Windows folder named Regbak).

Note, that by default, dat files are hidden, so you must have Show All Files enabled to find these files.  To do that:  In any folder, click View>Options>View and then check Show All Files. 

Restoring

Should the registry become corrupt, you can use your backed up files to restore it.  To do that, you would restart your computer in native DOS and at the C:\> prompt, type the following commands, pressing Enter after each one (in this example, it is assumed your backed up reg files are in C:\Windows\Regbak)

attrib -h -s -r c:\windows\system.dat
attrib -h -s -r c:\windows\user.dat
attrib -h -s -r c:\windows\regbak\*.dat
copy c:\windows\regbak\system.dat c:\windows\system.dat /v /y
copy c:\windows\regbak\user.dat c:\windows\user.dat /v /y

Once the files are copied, reboot. 

Scanreg in Windows 95 

Scanreg from Windows 98\98se can also be used to back up\ restore the registry in Windows 95. 

Simply copy the 98 files, scanregw.exe and scanreg.exe to your 95 hardrive. Both are self executing files.  Double click scanregw.exe and your registry will be scanned and then you will be given the option to back it up.

The first backup you make will be placed in c:\windows\sysbckup in a cab file named rb000.cab.  See the details for scanreg in the Windows 98 section for more details.

Windows 98\98se

Registry Checker - Auto Backup & Restore

The Registry Checker (aka ScanReg) is a built-in Registry backup and restore tool and comes in two versions -  scanregw.exe which runs only in Windows and  scanreg.exe which runs only in DOS. 
 
SCANREGW.EXE
 
The Windows Registry checker (scanregw.exe) scans your Windows 98 Registry for errors once a day. If there are no errors, the Registry Checker automatically backs up the Registry. If an error is found, the Registry Checker automatically restores the most recent copy of the Registry. The Registry Checker keeps, by default, five backup copies of the Registry and stores them as .CAB files in C:\Windows\Sysbckup. The first backup is called RB000.CAB, the second RB001.CAB and so on.  You can increase the number of backups created by editing scanreg.ini and changing the MaxBackupCopies entry.
 
SCANREG

To back up your registry in DOS using scanreg, restart your computer in DOS mode and at the DOS prompt type:  scanreg /backup  and press Enter.  Scanreg will back up your registry and then return a DOS prompt. 

Restoring

In most cases, you will be restoring the Registry in DOS mode using scanreg.exe.

Start Windows in native DOS mode - the easy way to do this is to simply choose Restart in MS-DOS Mode from the Shutdown menu or if you can't get into Windows, reboot. press F8 at the startup screen and then choose Command Prompt Only from the 98 Boot Menu.

At the prompt, type scanreg /restore and follow the prompts.

You'll see a box  listing each backup, the date each was created, as well as information that tells you whether the backup has been used to successfully start the operating system. All you need to do is select a backup and press Enter.

To see all your scanreg options, type scanreg /? at the DOS prompt. 
 
Manual Backup & Restore
 
You can also use the Registry Checker to  backup the registry whenever you choose.  Click Start>Run and type in scanregw.exe to start the Registry Checker. After scanning the Registry for errors, you will be asked if you would like to make another backup of the Registry.  Click Yes.
 
As well you can manually backup the 98 Registry by copying system.dat and user.dat which are in your Windows folder, to a safe place (same way as in Windows 95)  This is actually the easiest way to backup the registry.  It's just a matter of copying and pasting the two files to a new folder.  Note that these files are hidden by default.  If  you can't find them, open any folder and click View>Folder Options>View tab and choose Show All Files.

To restore the registry from your own backed up system.dat and user.dat, follow the instructions presented for Windows 95. 
 
Automating the Manual Backup & Restore of the Win9x Registry

The  entire manual backup and restore of the Windows 95\98\98se Registry can be simplified with a couple of bat files that when clicked will automatically backup or restore your Registry.  Go here for full instructions.
http://www.angelfire.com/on3/vxdoin2/RegBak.html

Windows Me

In Windows Me, the registry files are System.dat, User.dat and Classes.dat. These can be backed up manually but to restore you need Ms-Dos access.  Use the same instructions and commands as described in the Windows 95 section.

Scanreg is also part of Windows Me and like Windows 98, the registry is backed up automatically at startup. However, the  main restore feature of Me is System Restore which automatically monitors and records changes made to the essential Windows system files including the registry.  If your system becomes corrupt, System Restore allows you to undo (or "roll back") a change that caused instability in your system.

To use, Click Start>Programs>Accessories>System Tools, and then click System Restore.  

Click Restore my computer to an earlier time and click Next.  

In the calendar that appears choose which Restore Point to roll your system back to and click Next.  

You will then be prompted to close all programs and reboot. On restart, a confirmation screen appears. Click OK to continue using your computer.

The next time you start System Restore, you will see a third option, Undo my last restoration. This is available in the event that the Restore Point you rolled back to does not correct the original problem that you were having and so you can easily get back to the point in time that you started troubleshooting.
For more info on System Restore, click here.

Note that System Restore does not have a very good track record when it comes to reliability. There have been many reports of disaster using System Restore so use at your own risk.  Personally, I prefer to manually back up the registry files as I have always done since Windows 95.

Windows 2000

Automatically backing up the registry changed with systems built on the NT kernel platform - Windows NT4 and above.
Instead of automatic reg backups, Windows 2000 ships with the the  Backup utility for backing up the Registry. It's located in the the System Tools menu. When you start it, you'll see an item called System State, under My Computer. The System State is a collection of system specific data and as you will see includes the Registry.  Simply select Registry and click the Start Backup button.

Another method of backing up the Registry in Windows 2000 (and NT/XP) is by rebooting your system and when you see the Please Select the Operating System to Start message, press F8 and then use arrow keys to select the Last Known Good Configuration option and press Enter. This instantly restores the most recent copy of the Registry.  You can also use this method in Windows NT.

Restore

To restore the Win2K Registry, start the Backup utility, select the Restore tab, choose the backup media, and then select the System State check box. Note that this not only restores the Registry, but all of the System State data.

The real registry files (known as hives) reside in \WINNT\SYSTEM32\CONFIG and are named  DEFAULT, SAM, SECURITY, SOFTWARE, and SYSTEM (no extensions)  but they can't be backed up by simply copying and pasting -  you will get a file in use error message if you try.  However, there are utilities you can use to make the backup possible. My personal favorite is ERUNT.  Just do a search on the net for more info.  Of course, if you have a dual boot fat system such as Windows 9x and Windows 2000, you can always copy the 2000 registry files from your Windows 9x partition where you have normal access to the files and folders on your 2000 system.   It also  makes restoring the registry a simple copy and paste operation. Just let Windows overwrite the existing files. And you can also always use the copy command in Ms-Dos or the Recovery console to do backups and restores. See the copy method for Windows 95 for an idea on how to use it.

Windows XP

Like Windows 2000, the registry is not automatically backed up in XP and the registry files are also named DEFAULT, SAM, SECURITY, SOFTWARE, and SYSTEM and cannot be backed up in Windows by copy paste operations.
They are located in \WINDOWS\SYSTEM32\CONFIG (a quick way to access the folder is  by entering CONFIG in the Run box) The same manual backup possibilites as in 2000 apply to XP.  You can use a utiltiy like ERUNT or copy manually if you have a Fat dual or multi boot system where you have access to your other system partitions.  When doing manual backups, make a point to keep them current. Backup regularly.
Windows XP's main backup feature is System Restore which backs up and restores your whole system.  However, like the Windows Me Restore utility, it does not appear to be overly reliable - there have been quite a few reports of it failing to restore.  Having said that, if you would like to use it, you can access it by clicking Start>All Programs>Accessories>System Tools>System Restore.  Here you can set Restore Points or do Restores to an earlier time.  Just follow the instructions which are very easy to follow. If you need more information on System Restore, click Start>Help and Support and type SYSTEM RESTORE in the Search box.

And also like Windows 2000, you can restore to the last known good configuration if you run into problems.
This refers to the last settings you used that had no problems.  To do such a restore, restart your computer and tap F8 until you see the Windows Advanced Options menu.  Here you will see where you can select the Last Known Good Configuration by using the arrow keys to select and pressing Enter to initiate the restore.

At the run line type this in, cmd

when you see the dos looking box type this in,

sfc /scannow

this will replace the system files.

A. More than likely, your machine was infected by the system32.exe virus, which your antivirus software removed without removing the startup entry in the registry. To resolve this error, perform the following steps:
 

1. Start a registry editor (e.g., regedit.exe).
2. Navigate to the HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run registry subkey.
3. Select the entry "Explorer.exe C:\Windows\system32\system32.exe," then click Delete (the path for this registry entry might be slightly different on your machine, depending on where you installed Windows).
4. If you see a registry entry for cmd32.exe, remove it as well.
5. Click Yes to the confirmation dialog box.

 If you can't find the registry entry I describe in the second step, review this FAQ for a list of other registry locations you can search.

Start-programs- accessories-system tools- Maintenance wizard

XP = C:\WINDOWS\SYSTEM32  run chkdsk.exe /f (like scandisk)

www.driverguide.com

www.driverzone.com

www.drivershq.com

driver and patch updates

www.softwarepatch.com

Win98 & WinME

Start-settings-control panel – Add/remove programs – startup disk – click Create disk

Get CD-ROM driver : usually in C:\

If CD driver is not on startup go to config.sys and add to first line

Device =(your CDROM driver name) /D:MSD001

XP

http://support.microsoft.com                                     Q310994

www.protonic.com

www.5starsupport.com

www.Helponthe.net or www.techguy.org

www.computerhope.com

www.microsoft.com

Remote

www.pcpitstop.com

www.pcsupport.com                      $

www.pcpinpoint.com                    

www.metaquest.com

www.liverepair.com                        $

Click F5 while computer is booting up to select safe mode

Win95/98 kept in windows directory called (username).pwl

Delete this if you forgot your password.

1. Open User Accounts in Control Panel.
2. Click your account name.
3. Under Related Tasks located on the left side of the window, click Prevent a forgotten password.
4. In the Forgotten Password Wizard, follow the instructions as they appear on the screen.

Win98/ME/XP

Run – msinfo.exe

Win9x/ME

            Winipcfg – IP configuration

XP

Ipconfig – IP configuration

To start File Signature Verification, click Start, click Run, type sigverif, and then click OK.

You can use the Error-checking tool to check for file system errors and bad sectors on your hard disk.

Open My Computer, and then select the local disk you want to check.

On the File menu, click Properties.

1. On the Tools tab, under Error-checking, click Check Now.
2. Under Check disk options, select the Scan for and attempt recovery of bad sectors check box.

To use ClearType for screen fonts

Open Display in Control Panel.

On the Appearance tab, click Effects.

In the Effects dialog box, select the Use the following method to smooth edges of screen fonts check box.

Click ClearType in the list.

Notes

• To open Display, click Start, click Control Panel, and then double-click Display.
• ClearType is ideal for portable computer and other flat screen monitors. ClearType may appear slightly blurry on desktop computer monitors.
• Whether you select Standard or ClearType from the list, you must have a video card and monitor that support a color setting of at least 256 colors. Best results are achieved with High color (24-bit) or Highest color (32-bit) support. Click the Settings tab to set Color quality.

We must now edit the Windows Registry to tell Windows where to look for the CAB files.

Click Start, Run, enter regedit, in the Run window, and click OK.   This will open the Registry Editor shown below.  Before proceeding Click Help, Help Topics, and select Restore the Registry from the Contents.  Record this information so you will know what to do if something goes wrong from editing the Registry.  If you follow these instructions and do nothing else with the Registry, it is unlikely that anything will go wrong, but never underestimate "Murphy.

Click on the Plus box to the right of the HKEY_LOCAL_MACHINE to expand the folder and continue expanding appropriate folders as shown until you see the Setup folder.

I have edited the below graphic to eliminate folders so that the pertinent ones can be displayed in one graphic.  There are actually many more folders between HKEY_LOCAL_MACHINE and Setup.  You will have to scroll down the window pane to display the relevant ones

.

Double-click SourcePath in the right pane (you may have to scroll it to see the entry) to open the Edit String window.  Edit the Value data: to tell Windows where to find the CAB files as shown, Click OK, Exit the Registry Editor, and Restart Windows.  Done.

Need to find out where programs launch themselves from? Check out the following or if you want to know for without a lot of searching. A good solution is to download AutoStart Guard, the Freeware Autostart Viewing, Alerting & Control System for Window.

·         RunServiceOnce subkey : designed to start service programs before user logs on and before other registry subkeys start.

o        HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\RunServicesOnce registry key

o        HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\RunServicesOnce registry key

·         RunServices subkey : loads immediately after RunServicesOnce and before user logon.

o        HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\RunServices registry key

o        HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\RunServices registry key

·         Run subkey : The Run subkey in HKLM runs immediately before the Run subkey in HKCU.

o        HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Run registry key

o        HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run registry key

·         RunOnce subkey : primarily used by Setup programs. The HKLM subkey version of RunOnce runs programs immediately after logon and before other registry Run entries. The HKCU subkey version of RunOnce runs programs after Run subkeys and after the Startup folder.

o        HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\RunOnce registry key

o        HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\RunOnce registry key XP also has RunOnceEx:

o        HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\RunOnceEx registry key

·         RunOnce\Setup subkey : specifies programs to run after the user logs on

·         Explorer\Run subkey :

o        HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer\Run registry key

o        HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer\Run registry key

·         Userinit subkey : there is an entry for userinit.exe but subkey can accept multiple comma- separated values. Can't find where program starting? Look here.
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\Winlogon\Userinit registry key

·         load subkey :
HKEY_CURRENT_USER\Software\Microsoft\Windows NT\CurrentVersion\Windows\load registry key

·         All Users Startup folder : very common place to find autostart programs for whoever logs on
Documents and Settings\All Users\Start Menu\Programs\Startup folder

•  Startup folder : the most common location for programs to automatically boot from
  Documents and Settings\user\Start Menu\Programs\Startup folder If you migrated from NT, the path is Profiles\user\Start Menu\Programs\Startup

A partition can also be converted after Setup by using Convert.exe. For more information about Convert.exe, after completing Setup, click Start, click Run, type cmd, and then press ENTER. In the command window, type help convert, and then press ENTER.

It is easy to convert partitions to NTFS. The Setup program makes conversion easy, whether your partitions used FAT, FAT32, or the older version of NTFS. This kind of conversion keeps your files intact (unlike formatting a partition.

To find out more information about Convert.exe

1.            After completing Setup, click Start, click Run, type cmd, and then press ENTER.

2.            In the command window, type help convert and then press ENTER. Information about converting FAT volumes to NTFS is made available as shown below.

Converting FAT volumes to NTFS

To convert a volume to NTFS from the command prompt

1.            Open Command Prompt. Click Start, point to All Programs, point to Accessories, and then click Command Prompt.

2.            In the command prompt window, type: convert drive_letter: /fs:ntfs

For example, typing convert D: /fs:ntfs would format drive D: with the ntfs format. You can convert FAT or FAT32 volumes to NTFS with this command.

Important  Once you convert a drive or partition to NTFS, you cannot simply convert it back to FAT or FAT32. You will need to reformat the drive or partition which will erase all data, including programs and personal files, on the partition.

This is part of the Reinstall and Reformat XP FAQ.

Here's how to wipe a hard drive that uses NTFS as a file allocation system. (If you don't know what that is, go here.)

You can do this one of two ways by using the Windows XP startup disks or if your computer supports a bootable CD-ROM -- most computers built since 2001 have this feature -- use the Recovery Console tool on the Windows XP CD-ROM. More on that shortly.

First, remember that reformatting a hard disk deletes all of the data that is on it, so make sure that you back up your data before you reformat the disk. That data is unrecoverable afterwards. It's gone forever.

To use the Windows XP Recovery Console to reformat a drive, follow these steps:

Start the computer by using the Windows XP CD-ROM (in the CDROM drive) or the start-up disks (in the floppy drive).
You need to go into your BIOS to change the boot sequence to do this making your CD-ROM drive your primary boot device. (How to do this.)

Once the computer has booted from the CD you'll see a "Welcome to Windows Setup" screen appear and after all the drivers have loaded (shown along the bottom), press F10 or wait and choose to use the recovery console (choose item #2).
In the "Recovery Console" (learn more about that that is at Microsoft.com click here), specify the appropriate Windows installation. It will be listed. It's probably #1. Then log on by using your Windows XP Administrator password. You are probably the administrator and the password if you have never assigned is probably blank, so just hit the Enter key. If you run Windows XP Pro and want to turn off the password prompt then learn how here.

Next, type:  map

and hit the ENTER key.

Note the drive that you want to reformat. The drive letters may be different in the Recovery Console from what they are in Windows XP, but if you want to reformat the C: drive then its likely the largest.

Next, type:  format C: /fs:ntfs

Hit your ENTER key.

Next, type:  y

Then press ENTER to continue. When the formatting process is finished, type

exit

and then press the ENTER key to restart the computer.

If this column still doesn't fully help you with questions about reformating or if you need personalized help with a problem, please see: Emergency Help.

Go to HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Uninstall

delete the key that are in error