Try using Event Viewer to look for clues, How To Use Event Viewer - http://www.bleepingcomputer.com/forums/t/40108/how-to-use-event-viewer/
Post the error messages reflected if/when such appear.
Check that all fans are working properly and assure yourself that the system is not overheating. Heat is a common cause of abrupt freezes.
Run chkdsk /r on the C: partition.
Why Chkdsk -r Instead Of Error Checking - The Elder Geek on Windows XP - http://www.theeldergeek.com/forum/index.php?showtopic=30751
If you choose to start chkdsk from My Computer/Explorer, put check marks in both options (scan, recover).
Edited by hamluis, 12 June 2008 - 03:03 PM.