Win2000 keyboard startup problem

Hi, I'm at off-topic risk here, but I've been hunting down this problem
for some time and thought I could try here.

On some (four) Win2000 Professional machines I manage, it happens very
often that, when the login screen comes up, the keyboard is completely
frozen; you cannot write anything, and even caps-lock, num-lock & c. have
no effect on the keyboard's LEDs, and of course also Ctrl-Alt-Canc does
not work. You can only reboot the machine by accessing the appropriate
option using your mouse.
After rebooting, everything is ok.

This not only happens after a cold boot; it also happens after a simple
reboot.
The keyboards are plain simple ones, made by different brands, with no
drivers or anything.

Now, these machines are configured exactly like other machines in the same
office, with very few hardware differences. One unique feature they have,
with respect to the others, is that they have NVIDIA video cards (an old
model, RIVA TNT2 Model 64); the others are ATI.
Win2000 has some builtin drivers for these cards, and it seems that the
problem happens more often with the (recent) official NVIDIA driver than
with the Windows drivers; but it still happens.

Has anyone here experienced the same problem? Any clues?
Thanks a lot.

--
Ciao,
Marco.

...."Have a Little Faith", Bill Frisell 1993