Here is what I posted a couple years ago and it seemed to resolve the issue. Hope it helps.
Follow this step by step, while you are in Windows. This also works whether it is a Magic or Mighty mouse and whether it's for W7 or XP.
1) Switch off the device you want to pair
2) Select "add a device" in the Bluetooth menu
3) Switch on the device (after few second it should get detected by windows)
4) Now, right click on the device and select properties. You should now get a window with the different services.
5) select the HID drivers for keyboard and mice and click OK (this is the important part)
6a) In windows 7 it should setup mouse and keyboard without any additional input.
I found that cure posted by S_ Eye at
http://discussions.apple.com/thread....art=0&tstart=0