Probably a bad power supply... That's usally the cause, but it could be lots of things. You didn't tell us WHEN is started restarting. Games, just sitting there, what?
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Desktop * Athlon X2 4200 | 2G RAM | 160G HD | 7600GT
Laptop1 * C2D T5550 | 3G RAM | 120G HD | Intel x3100
Laptop2 * C2D T5250 | 2G RAM | 160G HD | Intel x3100
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