He has a hardware problem, either memory which is most likely or hdd probs.
Win 2000 and xp will certainly show up any hardware problems the trick is to find out which.
Change his ram, take 1 out try it in a different slot, try another hdd, take all the pci cards out except for the graphic card, you can always replace them later.
Make sure that there in no card in the next slot from the graphic as this can cause overheating of the graphic card unless he has onboard graphics.
The cpu might be overheating, all of these can cause similar symptoms.