My pc turn off automatically

Hello,
my pc turnoff automatically.
once when my pc turns on it will work for 2-3 hours then it turns off automatically then again on turning on it will not be working even for 5 min(it will again turns off automatically).
i tried by re-assembling the hardware and found that
when i remove ram and on the pc and keep it for 2 min and then off it.
now,when on keeping the ram in the slot and then again turning on the pc will work again for 2-3 hours then again the same process is occuring.
so please any one can say me what could be the problem.
i even tried scanning my pc with free antivirus.