Go Here and download WIPE OUT, the instructions are in the 'ReadMe'. Wipe the HD clean.

Download Memtest86 v3.2 ISO image (zip) Here and burn to CD, boot the CD and check your memory.

Disable everything in the bios (onboard peripherals) thats not being used.

Dose the PC have a video card or onboard video ? either way try another video card.

Make sure you're using a good keyboard and mouse.

HD might be bad ????

The PC should run XP.