Can be caused by a nymber of things.
1. Spyware and viruses do a scan, what antispyware and antivirus are you using?
2. Corrupt windows files,
Repair XP by replacing corrupt files
in the run box, press enter and let system file checker find and replace missing / corrupt file in windows and explorer.This will take awhile to complete and you may be asked for the CD to be in the drive if windows is not preinstalled on the hard drive.
sfc /scannow problems
3. Bad Memory
4. Hard drive problem
Check for errors and defrag your hard drives -- My Computer - select drive - properties - tool tab - Error checking / Defragmentation.
5. Heat problems
Motherboard Monitor (1,736K) click here Monitors temperatures etc.
6. Driver problems
Check you have the latest drivers for your devices.