Well i recently had a similar problem and it was a problem with a leaky capacitor on my motherboard.
Let me ask you something ... does your pc shutdown perfectly as in the processor fans and mouse lights (for optical mouses) get switched off. Well this used to happen to me and mabbe that could be the problem tho it is unlikely.
More likely it is a over heating problem so check that out. And try a virus scan while you are at it
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