Computer Hangs afer 4 hours of operation.

Computer P4 2.0
O/S: Windows XP Pro.
RAM : 256 MB
Mother board : ECS P4

The computer works fine for say 4 hours. Then the whole thing freezes.
Then when I do a cold boot. It hangs when runing the BIOS ( during the
intial phase of finding the harddisk etc).
If I shut down the computer for 20 mins or so it works fine past the
BIOS running stage, but hangs after the XP comes up.
If I keep the computer shut down for a day or so, it works fine again
for next 3-4 hrs and then the problem starts again.

I have changed the Hard drive, BIOS battery etc but the problem still
continues.

The computer is still a fresh install. 

What might be the problem. Is it the faulty processor?

Thank you
Kiran Math