lol, you know what, my computer was doing the same thing before I put vista on the computer, I have absolutely no idea why I even checked BIOS aswell, that wasn't the problem. I'm assuming now that there was a fault in the button and after a smashing the button a few times it fixed it.
Anyway enough about that, in BIOS, look around for an option called power on after power failure or something along the lines of that, turn it off aswell. Then have a look for a wake timer, some BIOS's don't actually have one.