plz plz plz i need help

i have a compaq presario sr5490an quad core win 7 ultimate x64 everything runs well but for some reason after time like 3 to 6 hours or less in just drops to the start up screen were u press f10 [bios] [boot screen] not sure what its called can anyone help me ps it seems to happen quicker if the pc is not being used left on desktop

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