Has it been moved, monkeyed with internally or any playing in the BIOS? WHat happened between it didn't work right and now? New programs? Any clues?
It sounds like power supply, motherboard or CPU at this point, but we would need to know more about the omputer, age, etc. to guess further.
Have you ever worked on a computer before? Are you wanting to? Or is this best left to your local computer shop?