http://support.apple.com/kb/HT2418
Also, from the "
Boot Camp Installation & Setup Guide"
Using Bluetooth Devices with Windows
Before you can use a Bluetooth wireless device with Windows on your Mac, you must pair the device with your computer while running Windows.
To pair a wireless Mighty Mouse or Apple keyboard with your Mac:
1 In Windows, open the Bluetooth Devices control panel.
2 In the Devices pane, click Add.
3 Follow the Bluetooth Device Wizard to set up your device.
a To make your Apple wireless device discoverable, turn the device off and then on again.
b In the passkey pane, select Let me choose my own passkey and enter a numeric code.
Wireless Mighty Mouse: Enter four zeros (0000).
Apple Wireless Keyboard: Enter six to sixteen digits.
Passkeys for wireless devices are used to encrypt the information transmitted from the devices to your computer. You dont need to remember the passkey.