When I went to turn on my computer this last weekend, the thing wouldn't boot. It hadn't been touched since the night before when It was working perfectly. When I press the power button, the fans (all 4) start, then stop after maybe about a second. I've paper clip tested the power source and its working fine. I would think that I probably just need a new mobo then, but what really bothers me is twice since it stopped working it has started and worked flawlessly until i turned it off after using it (after which it would not boot back up). I've taken everything apart and have set just the mobo with the cpu and the power supply on cardboard, and the same problem persisted. No boot. Any thoughts? A mobo that decides to work occasionally? It'd be nice to figure out what causes it to actually work occasionally, as money is tight.
It does sound like your mobo. I was having the same problem (among many others) with a new build of mine, turned out the motherboard was dead. What proccesor are you running? Maybe your board would be cheap