My HP Pavilion 5000 CTO with XP pro SP2 has been having this problem for awhile. Whenever I do anything that makes the computer work harder (defragmenting, total system scans, watching videos, and any time I multitask with many different programs); it will just shut down. I assumed it was a virus at first, but Nod32 would run and it would shut down before it found anything. Safe mode doesn't help. So I recently reformatted the computer but I'm still having issues. Any ideas on what could be causing this? Is it hardware or software?