Okay, it's a PS/2 connection. And both your K/bs are PS/2 only. That makes it harder to test different ports, lol.
(I don't suppose you have any PS/2>USB adapters lying around at all?)
You say you can still login with the k/b which presumably means it still functions outside Windows including the BIOS.
If it didn't work AT ALL outside Windows, I'd say your PS/2 connection has died. But since it only goes dead inside Windows, that points to software vs hardware.
Have you tried safe mode? (press F8 during the POST/boot up period before
If it works in safemode, then it really points to some sort of software/driver
If it doesn't work in Safe Mode (which uses the generic default driver) - then we've got a problem.
Try Safe mode and report back.