I go with the PSU theory as well. Had a incident almost like that a few months ago. I had overclocked my P4 a bit, and everything was fine. Computer was idle and late in the evening I was about to shut it down when I noticed it already was off. I was a bit tired and thoght I had turned it off earlier. Next morning it did't boot up. Dead. PSU had obviously died the "idle death"

. Got a cooltek 500w and quit all OC. Now it's fine and more quiet than ever.
Cheers Porphy