This is completely hearsay, but I have heard that the adapter that comes with the MS bluetooth desktop stuff doesn't support all the various bluetooth profiles, only HID for a mouse and keyboard. At the very least, I doubt it comes with driver support to do what you are asking. After you install it, do you get a "Bluetooth Neighborhood" icon on your PC laptop?
I have successfully paired a PC and Powerbook using a 3rd party bluetooth adapter that I bought at CompUSA for about $40 for the PC. I'm sure that would still work with your mouse and keyboard too. You might have to go that route.
You might try to find a more PC savvy forum to find out more info about the bluetooth desktop though, since I doubt many people here are using it as MS intended..