You can see how to back up your registry in Windows here. This is done to restore the system in case you make any wrong modification in the registry.

So see how to restore your windows registry from the backed up file:
If your system doesnt open due to registry problem, reboot the computer and press F8 multiple times on boot.
Then will bring up some options. Select “Open Windows in Safe Mode”
When the windows loads, click start, then click run
Type regedit and press enter
Then select file, then select import
Now import the backed up registry file from the place where you saved it. (see the tutorial for backing up your Windows registry)
After you import your registry file, all wil be ok. Restart your computer after that and registry will be restored.

See more Windows Xp Tips and tricks, registry hacks for enhancing performance and helpful free tutorials for windows here.