You say that the pc reboots even during the post, so i believe that we can completely exclude the possibility of a driver fault... So this is probably an hardware fault were talking about, falty psu's often result in sudden reboots, actually, as far as i can tell, apart from bsod's, falty psu's are the major cause to sudden reboots but there are plenty of other problems that can cause such problems though almost all (when it comes to hardware) are all related to energy problems like damaged drives or short circuits. But lets not get ahead of ourselves and try to pinpoint the problem.
The fact that your pc reboots even during the post "is an advantage for us" since you dont need to get to windows to test every time you make a change so you can disable some drives or all of them and just run the bios on startup and let it stay there for a while to see if the problem persists.
If i were you i would unplug all the cables from your hard drives, cd/dvd drives, take any device that it's not essential to the start up like sound cards, tv cards etc, even the graphics card in case you have an onboard you can use, also, use only one ram module, then just turn the pc on, go to the bios and let it stay there to see if the problem remains, if it does try switching the ram module or switching slot, if it still persists then its probably a motherboard problem, cpu or psu (i would bet on the psu), if the problem goes away, repeat the test as you install every device so when the problem comes back you'll know which device is malfunctioning.
And sorry for the bad english...