|A number of possibilities.|
A faulty power supply
Some other hardware fault.
You make no mention of the Blue Screen of Death (BSOD) which suggest to me that you have automatic reboot enabled.
You need to go into Control Panel > System > Advanced > System and Recovery Setting and make sure that automatic restart is un-ticked.
Reboot and next time it fails you should get a screen with lots of information on it. This will give you a clue to the problem. Report back what you find and more help with be forthcoming.
If it still restarts without the BSOD then the problem almost certainly lies with the power supply. The only way to check if it is the power supply is to substitute it with a known good one.
message edited by StuartS