I owuld also Google for the ATX pinout. One of the pins is called "power good" and it is supposed to tell the motherboard that the PS is in regulation and ready to go. If it does not do that, there will be no boot.

I would measure all the supplies because in a switched mode power supply like that, all the sources' levels are chiefly determined by the secondaries of the transfomrer, but only one is actually in the feedback loop. There may be subregulators on some supplies, there usually are, but there is a good possibility that when one supply drops because of poor filtering, that other sources will rise because it has affected the feedback loop. That can fry **** out.

It's about lke the PSes in the older VCRs where the 5 volt line would drop slightly, and the regulation would make it crank out voltage until the 12 volt line hit like 17 volts. at 16.9 volts, everytihng that dissipates pwoer on a 12 volt line dissipates twice as much. That can fry **** out.