I would at first check if some software running in the background is causing this. See "How to Troubleshoot By Using the Msconfig Utility in Windows XP" http://support.microsoft.com/?kbid=310560) for
details. Also run a disk check.
"How to Perform Disk Error Checking in Windows XP"
(http://support.microsoft.com/?kbid=315265)

Also, have you unistalled any software recently?