How to Edit the Registry

To open Registry Editor, click Start, click Run, type Regedit, and then click OK.
NOTE: You should not edit your registry unless it is absolutely necessary. If there is an error in your registry, your computer may not function properly. Incorrectly editing the registry may severely damage your system. Before making changes to the registry, you should back up any valued data on your computer.

Back up the Registry First!!!

Windows 95, Windows 98 & Windows Me:

NOTE: Do not follow these steps to export a whole registry hive (for example, the HKEY_CURRENT_USER hive). If
you must back up a whole registry hive, back up the whole registry instead.
Click Start, and then click Run.
In the Open box, type regedit, and then click OK.
Locate and then click the key that contains the value that you want to edit.
On the File menu, click Export.
In the Save in box, select a location where you want to save the .reg file, in the File name box, type a file name,
and then click Save.
To Back up the whole registry:
Click Start, then click Run. The Run dialog box appears.
Type regedit and click OK. The Registry Editor opens.
Click the Registry menu and click Export Registry File.
Verify the following items in the Export Registry File dialog box:
Save in: Desktop
File name: Registry Backup
Save as type: Registration Files
Export range: All
Click Save.
Exit the Registry Editor.
Verify that an icon labeled "Registry Backup.reg" is on the desktop. CAUTION: Do not double-click the "Registry Backup.reg" file on the desktop unless you intend to undo the changes you are making to the registry.
NOTE: This method for Windows 95/98/Me will not work for Windows NT/2000. Windows NT/2000 has additional security protection that prevents this backup from being restored.

How to back up the Windows 2000 registry

http://support.microsoft.com/default.aspx?scid=kb;EN-US;322755

Windows 2000

NOTE: Do not follow these steps to export a whole registry hive (for example, HKEY_CURRENT_USER). If you must back up whole registry hives, back up the whole registry instead. Click Start, and then click Run. In the Open box, type regedt32, and then click OK. Locate and then click the key that contains the values that you want to edit. On the Registry menu, click Save Key. In the Save inbox, select a location in which to save the .reg file, type a file name in the File name box, and then click Save.
Windows xp Home edition does not include the Backup utility by default. To install it:
Insert your Windows xp cd.
Open up Windows explorer.
Navigate to X:\VALUEADD\MSFT\NTBACKUP\ (replace X with appropriate cd-rom drive letter) Double-click NTBACKUP.MSI and follow the on-screen instructions.

Windows Xp & Windows 2003 Server

NOTE: Do not follow these steps to export a whole registry hive (for example, the HKEY_CURRENT_USER hive). If you must back up whole registry hives, back up the whole registry instead. Click Start, and then click Run. In the Open box, type regedit, and then click OK. Locate and then click the key that contains the value that you want to edit. On the File menu, click Export. In the Save in box, select a location where you want to save the Registration Entries (.reg) file, in the File name box, type a file name, and then click Save.

How to backup the registry in XP

http://support.microsoft.com/default.aspx?scid=kb;en-us;Q322756

http://www.puppypc.com/tutorials/reg/backup_xp_reg.htm