I'd say it's likely to either:
a) Power Supply - not enough power providing enough oomph for the game
b) Dodgy CPU - graphics/sound card is feeding it all this stuff it cant handle and just gives up.
c) Graphics card is kaputt.
Take your pick, cos I honestly don't know with such a short desc. Take it to a tech support dude.
everything red superfly has said is a very good point, another one i could add to that list is a registry problem. my old computer used to restart whenever it felt like because of that, maybe you should try research that one?
and another thing that made my games restart the computer was graphic card drivers. i have about 1 year old radeon 9600 drivers, and i updated to the brand new catalyst ones and it buggered by PC up. maybe it could be that too?
or you have spyware lol 😛
research the problem some more... email blizzard at
[email protected] if you cannot find it on their www.battle.net site with all the technical support FAQs... if all that doesnt work I'd recommend a reformat