This worked/is working bulletproof for me.
Unpair mouse and keyboard from Windows (if you've already got it paired and not working somehow.)
Unpair mouse and keyboard from OS X.
Boot back into Windows - Remove Apple Bluetooth and Apple Bluetooth Enabler drivers. Restart. Generic Windows drivers will be installed. Remove Apple Keyboard driver. Restart. Install the Apple Keyboard driver from an earlier version of Boot Camp (I used the one from Leopard 10.5). Restart. Now, do the standard bluetooth pairing.
You may or may not have to right click on the Keyboard/Mouse whilst it's pairing and go to "properties", then enable "Drivers for Keyboard, mice, etc." service under the service tab for the device.
Now, your keyboard and mouse should work always. Also, you should turn off power management for the bluetooth drivers as well as it will interrupt the connection.
Repair with OS X. And voilla.