Power-up, Power down; no luck

Need help. Computer was still running (fans) in the morning after hitting "shut down" the night before. Used rocker switch to kill power. Wouldn't start the operating system when hitting the power button. After awhile the system would come up but still wouldn't shut off the power to the fans and gosh knows what else. Took the computer to an idiot who wiped my hard drive (and all the files) and re-installed an operating system (Windows XP) but the problem remained. Replaced the power supply. Still no help. The front power button on the case was damaged so I ordered a new case and moved the components. Still have the same problem.
So far I have eliminated the Operating System,the power supply and the case.
What is wrong? Motherboard? Hard drive? CPU?
Please help.

Thanks,
Phil