Faulty Hardware?
I don't know if it would be overheating on just a boot up, unless the processor fan is fried :dead:
What I would check, until you get a better suggestion, is to remove all unnessary PCI cards and disconnect everything except floppy, keyboard, mouse, graphics card, and monitor. See if it powers up then. If it does connect 1 component at a time until it fails. This will also ensure that the cards are seated properly. If a component fails remove the last component, the one before the one that caused failure, while keeping the one that caused failure in. If it reboots then it could be power supply. If not then its possibly the card. See if you can check it in another computer.
-SNGX1275