Hmmm.. what brand make is your keyboard and mouse kit? I've never set up a system using wireless kb/ms, but I know that the Logitech Wave wireless kb/ms works in POST and BIOS.
You may need to enable "USB Keyboard Support" in the BIOS (if you can). Some boards ship with it off for some reason.
There is no reason why the wireless kb/m should not work in POST - the receiver does not require drivers, it simply creates a link between the keyboard and mouse - the signal entering the USB port is essentially the same as if you were typing/mousing with wired peripherals.
I would double check that your keyboard and mouse are connecting to the receiver properly. Maybe you could try it on another computer?
With windows XP-vista you can (not sure with 2000). Make sure you connect the battery the right way round - to - + to + . Then connect reciever to usb port. Then simultaneously Hold down the 'Connect' Button on both the Reciever and the Keyboard. Then do the same for the Mouse.
And my experience has been that it won't work that first time around. I always keep a spare kb and mouse handy for this reason. My wife really likes the wireless devices. They don't play nice with kvm switches either.
Stecman's right though, you should check the usb kb setting in bios.