I suppose you have tried a System Restore?
Other than that, have you tried running:
System File Checker (sfc /scannow)
In the RUN box on the Start menu, type, sfc /scannow
You will also need a copy of Windows XP in the CD/DVD drive for when windows wants to install or replace a file.
Probably not very relevant in this situation but there is a tutorial from the link below which explains how this works, also if you want to install the necessary files on a permanent basis, but due to the frequency and space it takes I don't think many people bother with it, but for troublesome PC's it can be well worthwhile because System File Checker can take a while to run and you will need to be at the PC to keep selecting the yes button especially if you have renamed the CD/DVD drive letter without changing it in the registry.
How to Run System File Checker (sfc /scannow) without a Windows XP disc
http://forum.kitz.co.uk/index.php?topic=3103.0