Like adavidw, I administer 30 machines and have five powersupplies which "failed" in the last, oh, 6 months. But they didn't fail, they simply overhead because their CRAP "sleeve" fans get dusty and die. Once I figure out how to solder, I'll swap those fans out for double ball bearing Antec fans and put the PSU's back in service.

I wouldn't think it's a bad cpu or motherboard. In the past 15 years, I've probably only seen three bad motherboard and one bad cpu. One motherboard's problem was inexplicable, one motherboard had physical damage, as did the other motherboard/cpu combo (thanks to me trying to install at 2am the heatsink which was CLEARLY designed to be as unergonomic as possible... requiring proprietary tools to install?)

Bad ram, maybe. I had an Abit motherboard that burnt out its own ram chips. Turned out, Abit decided to "speed boost" their ram by increasing the voltage higher than usual by default. After a year, the mid-grade ram was toast (though my machine is the same Abit, and the registered DDR chips have survived for years).

For a machine that sat idle, there's even a tiny chance the motherboard battery is dead. I've had machines which won't POST without it. Good luck.