[Klunk]

The BIOS will only read data from a USB K/b if that bios version has been recently produced. (2005+)
Plug it into a rear-mounted USB port that ISN'T on some sort of port adapter. Try not use wireless USB keyboards, for some reason, the BIOS doesn't read properly.
(If your really REALLY stuck, you can buy a PS/2 Keyboard for £5 =P)
Maplin's £5 PS/2 keyboard