Unwanted winows reboot

So I think the above pretty much describes my problem
Every other hour windows reboots. Any possible causes? (Nod32 didn't find anything)

And no, I didn't touch the power button

>>2
>>3

hmm.. this was the last thing I wanted it to be..
You see I left my computer at reparation and got it back a couple of weeks ago. I left it  because it had overheated
Yes, so badly that the graphics card showed little but graphical artifacts all over the screen, because it had supposedly melted..

>>4
I don't know..