eMachines, Gateway, HP, and possibly others bought cheap Bestec brand 250W ATX power supplies a few years ago, that are prone to failing after 1-3 years. They fail in such a way as to destroy the motherboard. If you're not familiar with the problem, try google emachines bestec power supply failure. Anyway, has anyone investigated how exactly these power supplies fail, to kill the motherboard, silently and without smoke? If so, do you also know what component(s) they commonly trash on the motherboard?
I've tested my faulty power supply (it ate two motherboards before I thought of doing the search above) and found all the outputs were right on, under a very light bench load. I didn't check the unit under full load on all the outputs, or for AC potential - I suppose the main switching transformer could have shorted the primary to the secondary, or an opto isolator might not be isolating anymore.