Computer will not boot to windows

I have a desktop (custom) that will not boot to Windows XP SP3. If I run CHKDSK from the windows recovery console I am able to get it to load for a few hours, then it will just shut down and not boot again. The shutdown is random with no pattern. When trying to use the Windows Boot menu the computer will shut down and not allow me into Safe Mode or Last known good.

I have tested all the hardware, power supply, Temperatures, fan speeds and flashed the BIOS - loading the most recent version.

I have run dianostics using the tools from ultimated boot CD on the CPU, the motherboard and RAM with no errors found.

CHKDSK does find errors and 'fixes them all'.

The harddrive has been formatted with a new copy of SP3 on it.

The computer has an AMD Semperon 3.6 processing with an ABIT NF-M2S motherboard. There are 2gig of ram and all components are onboard.

The computer has no floppy drive, 2 SATA drives and one IDE Drive.

Any thoughts as to what might be causing this and how to fix?