No way to be certain but I would test the following.
-Hard drive test
-Memory test or swamp with known good memory and see if problem persists.
-Video might be going bad. I've seen onboad video die before and had to use a PCI or AGP card in it's place. Try a video card to see if the problem persists.
-Maybe it's a bad PSU. Try another PSU. I made this last since it would be the most annoying to swap out.
I think bad CPU is very unlikely. But on that note, check to make sure the fans aren't clogged.
This CD might help - UMCD