I found these instructions to finally work... (after following them then rebooting).
Hello all.
I would like to thanks BeDammit for his brilliant solution.
I' ll add some extra information to help people flawless set up their Apple Bluetooth keyboard on Windows XP.
1. Press Win Start button, choose Run and then type regedit and press enter to access windows registry editor.
2. Into the registry editor search for and select -> HKEY
CURRENTUSER\Software\Widcomm\BtConfig\General
3. Set the key 'PinCodeWord' to a decimal value of 1111
4. Set the key 'UseFixedPin' to a decimal value of 1
5. Then attempt pairing. When you get to the greyed PIN screen type 1111 using OTHER keyboard THAN Apple Bluetooth keyboard.
6. Press the Return key on the Apple keyboard.
7. Enjoy your nice Keyboard.
I think this one is the best solution cause you don't have to mess with windcomm driver version.
Thanks again BeDammit.
Originally found at....
http://www.neowin.net/forum/lofiversion/index.php/t285546.html