Well, this is a non-linux question, so maybe a moderator can move it to another forum.
You might be having a heat related issue. Have you recently blown out the cooling fan and heat sink for the main CPU?
Have you recently run an antivirus scan, spyware scan, and defragmented the hard disk?
When was the last time you reinstalled XP? Every couple of years, I just up and reinstall Windows since it's easier than fighting with it...