[Archduke Tips]

I thought about this problem more today, and I think it is safe to assume that you have a hardware problem. It is just very unlikely that you would encounter a software issue that auto-restarts that isn't a virus.

Depending on your motherboard, any hardware device failing could trigger a restart. That leaves you quite a few parts to check, but I'll at least tell you what I think are the "usual suspects" to get you started.
1.) Check the overheating issue first
2.) Check the power supply - make sure it is on the appropriate voltage level (120Vac for US 240Vac for Europe)
3.) Check the power supply again to make sure it is generally working right
4.) Check that all wires and cards are plugged in firmly
5.) Check that your RAM is working (microsoft has free programs)

I can give you more details on how to perform each of the checks if need be. Just let me know how the overheating issue goes first. I think that is most likely what is causing your problem. The CPU probably overheats when it has to deal with all the garbage that windows loads.

By the way, if all the fans seem to be working properly, make sure it isn't just a dust issue. Heat can't transfer off the heatsinks very well if they are covered in dust. That's kind of a stretch though.