For a while now my main PC has been in the habit of freezing just after Windows 7 desktop appears, and all I can do is do a hard reset after which it boots OK. It has also very occasionally done the same much later on.
Today however it did the freeze, and when I tried to reboot it got a little way then just powered itself off. I tried a couple more times and each time it got less and less far until it wouldn't even complete the POST.
After leaving it for a few hours I tried again and it went a little way then stopped, then it was back to nothing.
The fans all fire up, so there is some power getting through initially.
I'm guessing it is either power supply or mobo, and I would pick power. My "spare" PS is actually too old tro run this PC. I do have a suitable PS in another machine, but I'm reluctant to strip that one down if I can avoid it.