could be several things.
Overheating, Bad memory module. faulty mother board, or a virus.
If it is a virus, there would be some message on the screen before it reboots or it would reboot only when you do certain things like opening up a command prompt or trying to access the registry.
You'll have to check for all of those conditions before deciding what the problem is.
I was quite sure it wasnt a virus! and then I checked the mobo.. and there it is.. 1 memory module wasn't working and later found out that not only the memory module but that particular mem slot also isn't working!
So yes! it could be many things.. maybe you can check your memory first... take off the mem and take an eraser rub it off the contacts to clean them a bit and try running it on with a single stick and if that doesn't help change slot and see..