Can u help me to solve this problem?

i have a trouble about my PC is that: each time i download everything, my computer will automatically restart, i don't know why, i tried to solve that:
1, i think it's a virus or something like that, but i don't get any warns, and i scaned with my software but there's nothing.

2.i also shutdown automaticlly restart in Xp
But it's still happen. would u mind helping me

THanks For Best Friends
























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