By default the Legacy is set to Auto, so the USB keyboard should work. Have you disabled the Legacy?
Thank you for your reply! I'm not sure, but can't think of any reason why I would change that setting since I had no knowledge of what it was for before my PS/2 keyboard broke. But then why doesn't my USB keyboard work before Windows XP starts? I have a basic Logitech K120 USB keyboard that I've tried connecting to all USB ports on the computer without luck. Based on what you write I probably did disabled USB Legacy. Perhaps by accident?