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.