USB keyboard not detected by setup

I recently replaced my old PS2 keyboard with a USB Microsoft Natural Ergonomic keyboard. I have a password set in setup which must be entered to permit bootup. (This is for security reasons.) The problem is that setup is not detecting the keyboard, so that I can't enter the password! Rather than plug the old keyboard in every time I reboot (or more likely the PC crashes and I have no option ) I have disabled the password except to enter setup.

I looked at setup but couldn't see any relevant options there for detecting a USB keyboard at this stage. Any suggestions please? You're going to ask me which BIOS it is. I think it's AMI but I'm not certain.

I did at one point find that once I had actually managed to get into setup (with the PS2 keyboard) it would now recognise the USB one, but for some reason this doesn't work at the password entry stage.