From what I�ve seen usually when the comp reboots out of nowhere it�s a hardware conflict or problem.
You could try to take out for example the network card, sound card or any other hardware that is not required to boot up the comp and install a fresh copy of windows. If everything works fine, add them one by one and see what happens.
Also you should check the RAM and the slots and make sure they are ok. You should also take a look at the CPU temp since if it is higher then 70 or 75 degrees usually the comp reboots and it keeps rebooting till the temp drops.
Good luck.
