Why does keyboard cease to work when trying to do reistall setup?

I've noticed many times that computers are very fussy about keyboards when doing reinstalls. I have both a USB keyboard and a PS/2. Neither one would work when it came time to press whatever key it was (F8 or enter?) to select to do the Windows Reinstall. The up and down arrow didn't work either. I finally had to physically turn the computer off or uplug it. I finally wound up actually using a dell USB keyboard that worked on a Dell Computer.