It could be a hardware issue but doesn't necessarily have
to be overheating. Have you added any kind of hardware to
your system lately? In particular a new video card could
cause some major havoc if you don't make sure your power
supply has enough "juice" to support it. Generally a 400
Watt P/S should do the trick but not many standard PC's
have it.
Another thing I have run into that looks like your problem
was RAM timing problems. I upgraded my PC at home to a AMD
3200+ and even though I had 3200 DRAM (400MHz) in my
system the computer kept rebooting. I took one of my
DRAM's out and ran it on one only - problem gone. Reading
up on newsgroups "afterwards" showed me that you pretty
much want 2 equal DRAM's in your system if you go to a
400MHz FSB.
Hope that helps but in any case - good luck
