Are all of your cooling fans working correctly and does your PSU supply enough power for all devices installed ?
This sounds more like hardware error than anything else, assuming that your system is clean of virus infection.
If you still have Windows XP installed, open up the Control Panel, double click on Administrative Tools then double click the Event Viewer. Single click on the Application, Security and System categories in the left hand panel and check the error logs in the right hand window.
You need to look for anything with a red circle containing a white plus sign or a yellow triangle with an exclamation mark inside. These are the errors as picked up by Windows and reading them could give some indication about possible causes.
Depending on the file format of your hard disk, you could also do a full disk check. If your hard disk is FAT32, boot from a Windows 98/ME startup floppy disk and run a complete scandisk, checking for damaged sectors. The NTFS equivalent is CHKDSK.