Did it come with a Bluetooth dongle/USB adaptor or do you have a bluetooth adaptor on your motherboard (in the computer)?
- If not, then you'll need to buy a Bluetooth dongle/USB adaptor
Then go to http://code.google.com/p/uawks
/ (unoffical Apple Wireless Suport), download the program (which is on the right side of the screen)
Link is "Install UAWKS (date seperated by dots).exe"
Make sure you select the one states "standalone, get this if you have never heard of AutoHotKey"
-EDIT also look at this post: http://www.tomshardware.com/forum/193169-28-does-apple-wireless-keyboard-work