Keyboard stops working when Windows XP starts

I recently replaced my PC case and upon Windows startup, the keyboard, a
Microsoft Internet Pro, no longer works. It works fine while in the BIOS
setup, but chokes when Windows' desktop appears. I should add that originally,
both the keyboard AND mouse were inoperative until I did a repair install of XP.
The mouse, an MS Explorer 3.0 optical on USB, has been working fine since the
repair. So far, what I've done is try 2 other working keyboards with the same
result, then I connected another hard drive to the machine, disconnecting the
old one and boot up with the XP that's installed on that one. Everything works
just peachy. Reconnecting the original drive reverts back to the original
problem - no keyboard. I've also uninstalled the keyboard in Device Manager and
rebooted and this would result in the keyboard working again...until I restart
the machine at a later time. My latest trick is to uninstall the keyboard, then
immediately go to Control Panel and add new hardware, which finds the errant
keyboard and installs the driver. At that point, the keyboard works again, even
though I get a nag that I should restart my machine for the change to take
affect. We all know by now what that will do.
So WTF is going on here?