try taking out all your pci cards (pci cards=network cards, 56k modem cards, sound cards), graphics cards (ONLY if your mobo has onboard), and try one stick of ram
if that still doesn't work, then run memtest64 (memtest and then a number, i forgot) and see what you get
if that still doesn't work, then run memtest64 (memtest and then a number, i forgot) and see what you get