Further to this, I have seen on more than one type of PC if the USB keyboard and USB mouse are plugged in the "wrong way around" neither will work. What I mean is that like the older PS/2 connections which are KB and mouse specific, it would seem the USB are the same.

I know this shouldn't happen since a USB port is a USB port but my Dell Dimension 4100 definitely does this - perhaps it is a BIOS thing. I haven't ever tried using them in the BIOS when their in the wrong ports though. The mouse is optical and the obvious symptom is that the LED won't light if plugged into the wrong port.