System Restore can fix bad application installs, updates, or many spyware invasions. System Restore allows you to go back to a prior time (before the incident occurred). System Restore will not remove or change any user data and many times can be a very easy quick fix.

I'm not going to re-write Microsoft's instructions, they're pretty clear and are easy to follow. See the list below for complete instructions.

How to Use:

http://support.microsoft.com/kb/306084

How to start System Restore in Safe Mode At the Command Prompt:

http://support.microsoft.com/kb/304449

How to TroubleShoot System Restore

http://support.microsoft.com/kb/302796

How to Restore the WinXP Registry (even has a video)

http://support.microsoft.com/kb/309340

How to recover from a corrupted registry that
prevents Windows XP from starting

http://support.microsoft.com/kb/307545