If it is in fact a hardware issue, the following list is ordered by decreasing likelihood:

1. Memory
2. Power Supply
3. HDD
4. Motherboard
5. CPU

In my 2+ years as an in-the-trenches hardware tech I've seen a total of *two* dead CPUs, and both of those were part of a classic PSU-Mobo-CPU death trifecta.

Good luck w/your diag!