Windows do not load after auto shutdown

Hello,
I am Rahul.
my computer is P4
Ram 1 gb
It suddenly auto shutsdown after 2 hours and when i try to start the pc again windows do not load and the screen only shows intel.There is also a beep sound along with the screen.
Then i start after 2-3 hrs till the time it is ok but the same problem occurs afterwards.
Kindly give me a solution for this
Regards.