I have the exact same problem. I just installed a new Geoforce 2 GTS-V video card...installed the latest nvidia drivers and all of the sudden windows 2000 won't shutdown or restart.
My problem is a mirror of crofty83's description. I have re-installed my W2K OS four times today because I thought is was an IRQ/ACPI issue. I had messed with my BIOS, registry, and ACPI in Device Manager. No Luck!
Finally realized that it was the nvidia drivers (I tried three versions). I can uninstall the drivers and "presto" my machine would shutdown and restart normally.
From what I have read the remaining of today is that some drivers/applications can cause problems like this in W2K.
Anyone else have issues with the NVidia drivers and your machine shutting down properly?