Open your case, and have a good look at all the capacitors on your motherboard. If you have the older style capacitors (not the solid state ones), if any of them have a bulging top or are leaking, then that would be the problem. The aluminum tops of the capacitors should be perfectly flat.
If you have swollen capacitors, then the motherboard would have to be replaced. I've heard that you can replace them by soldering on new ones but I haven't had luck with that so far.