I have a microsoft USB keyboard that I've used on many linux machines-- on my Ubuntu 12.04 machine, I can boot the machine, hit the key, and go into the bios menu just fine. The keyboard works fine, I can switch through the BIOS menus with no problem.
When I quit the BIOS setup and allow the machine to boot into Ubuntu, the keyboard doesn't work at all. I can't use any hotkeys or ctrl-alt-f1 maneuvers.
BIOS does have USB legacy mode set to enabled.