As far as I understand while both, your USB keyboard and PS/2 keyboard, do work fine in the Operating System, the same keyboards are not recognized during POST, and, consequently, you are not able to boot into BIOS. If my inference is correct, then, I'd recommend the following:
1. Try one keyboard at a time, during POST.
2. As for the USB, test it out on each of the available USB ports on the back panel:
3. If still, you cannot get it to POST into BIOS, then, clearing the CMOS as outlined here may help.