ok, so what you need to do is get widcomm drivers and some dongle that they refer to in this forum to get it working:
here is a guide that is posted in that thread to switch your driver
and then I think the most important post on the first link is this one:
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_CURRENT_USER\\Software\\Widcomm\\BtConfig\\Ge neral
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.
hope this works for you, if not let me know and I will do some more research for you