"Upgrading" over a previous install is nearly always a bad idea. 3rd party drivers can be "successfully" migrated but not really compatible; double check you have the newest W2K-certified drivers for all your hardware. Also, subtle registry errors can be pulled along as well--a million things can go wrong.
That said, did you install all the fixes available via Windows Update? Can you shut down successfully?
If so, it's probably a driver issue which is why it's important to get all those updates. Also, check the Advanced Power Management and try turning it off.
APM is off. I'm going to reboot now and check the bios.
I seem to have shutoff problems myself every time I upgrade. One time it was burner drivers, another time it was network settings left over from a card I had pulled that caused the problem. It's not a problem confined to the W2k line, but a more general upgrading issue - though the actual conflict causing the problem here is perhaps W2k specific. Without knowing what he has installed and how it is configured, it is pretty difficult to diagnose precisely. Getting the latest W2k drivers for everything is a good idea. A clean install is even better.