Win 2k Blue screen

First - sorry I don't have exact details. A co-worker brought in his W2k machine because it would not boot just blue screen. The screen said something about any new added hardware or a possible virus - found neither to be true. Googled it and found chkdsk clears it out. Sure enough I used Barts disk and chkdsk and the system boots fine... for a while..
I have verified there is no new hardware, ran many virus scans all come up clean, the hard drive passes diagnostics... but this error slowly creeps back in.. I gave him the bootdisk so he could fix it when it happens again (it does).

One more clue the systems slows gradually then dies - the moniter goes from green to flashing amber - the PC goes off.
Timeline is weeks.. not hrs or days...

Anyone heard of a permanent fix? Got a clue what the real problem is?
Possible heat issue - the 'wind tunnel' on the processor is gone - he has a large case fan pointed to the CPU.. it is a 800Mghz.