I'm actively searching for a solution, but I have yet to figure this one out.
I recently purchased an older Cherry model keyboard which is PS/2 (via a DIN->PS/2 adapter). The keyboard works just fine on my old XP SP2 computer, but it won't be recognized beyond my BIOS on my Win 7 64bit machine. My mobo (ASUS Maximus IV Gene-Z) has one common PS/2 port but has no PS/2 settings in the BIOS; the keyboard works just fine in the BIOS. Upon loading Windows any lock LEDs clear and no input is recognized.
The keyboard appears on my Device Manager as a HID Keyboard across USB which is bizarre since it's connected via PS/2, so I'm leaning towards it being a Windows problem. The keyboard isn't recognized either when using an active PS/2->USB adapter.