Today I re-installed my old PC for my sister. First obstacle? I obviously had changed the boot order to put the CD Rom Drive after the harddrive, making booting from my Windows install DVD hard, to say the least. I couldn't change it because, for some reason, the BIOS refused to recognize my USB keyboard. And unfortunately I seem to have no more PS/2 keyboards.
After a bit of trial and error, it seems that only one of the USB ports on the rear is recognized during POST. And on the Asus P5QD Turbo mainboard it's the port directly next to the on board NIC port:
So if you have this issue with any Asus board, try that USB port first. Of course if you have a PS/2 keyboard handy, that'll do the trick as well.