Why does my computer keep restarting?

Could be a virus. Could be a number of things.

Overheating? If it's a laptop make sure to never actually place it directly on your lap as this traps in all air and block all fans, if it's a desktop try cleaning out all the dust in it and making sure all the fans are working. If its an older Desktop and its over heating could need some more heat sink for the CPU. If it gives you an error explanation on restart let us know what it says.

It's probably a virus. Get AVG Free and run a free scan. Get it here> http://download.cnet.com/AVG-Anti-Virus-Free-Edition-2012/3000-2239_4-10320142.html?part=dl-avg_free_gb&subj=dl&tag=button

That's a little like asking "why doesn't my car start" without saying whether it turns over the engine, makes noises or could be out of gas. Don't be afraid to give too much information about your issue. In a case like yours, the more info you give the better help you will get solving the problem. Have you recently added new software or hardware? Is your PC up to date with MS critical updates? Stuff like that along with a description of what happens (does it get so far and then restart, can you get into safe mode) would help us help you tremendously.