Is it restarting repeatedly as soon as the computer loads, or does it do it at seemingly random times (like once you've gotten to working on the computer for awhile, for instance)? If it's the latter, it sounds sort of like a "Joke" program (like Joke.Restart) that acts mostly as an annoyance; the former would be more damaging and doesn't match anything I can think of offhand. Of course, I'm definitely not the most knowledgable computer person here; I'm an okay programmer (well, kind of), but I'm not nearly as good with this stuff as a lot of people.
It might help to know what OS you're running, if you don't mind mentioning it here. I assume you've tried some sort of antivirus scan on your own (as long as you can do so before it restarts), right? That kind of info also might help the geniuses in helping you.