My desktop spontaneously powered down?

[dmorin at lear.morinfamily.com (Duane Morin)]

I just went up to my office to discover that my PC
was powered off.  Odd, since I left it on.  Power was
flowing to the power strip, and the other devices on
it including printer and monitor.  Oddly the PC appeared
to still be getting power, as a green light on the front
of the case was glowing.

I wiggled a few cables and noticed tat the green light
was out.  Assuming I had just noticed it at the wrong angle,
I tried the power again, and the PC came on.  Stayed on
long enough to fire up XP and let me log in, but a few
minutes after that, shut itself off again.  Now I can't
get it to turn on.

This is completely new behavior for me.  Ideas?  I'm
wondering, it's a hot day, could it just be overheating
and shutting itself down?  And the first time I caught
it right when it had cooled off enough?

Could my power supply be dying?

Just to rule it out is there anything that could run
on a PC (i.e. virus) that could shut down my machine?

Duane