It could be hardware related if you haven't done anything to the software like adding software, removing software etc...

I'm kind of thinking it might be software if you had problem with system files. You might want to try running the Xp system file checker:

start, run, type sfc /scannow

Make sure your Xp disk is handy. That should find and replace any damaged or corrupted system files.

You might also want to check your ram, motherboard settings, any third party PCI cards, your power supply (is it adequately providing power), etc...

Also check your error logs in the administrative tools. Should be under event logs. See anything there? If so post it up here. It might help trouble shoot this problem and bit easier.