Windows doesn’t have the drivers by default. They are bundled with Boot Camp. Installing Boot Camp will also enable the special function keys, like volume up and down.
1. Download and install Boot Camp 3.0 from:
insanelymac.com (look for M4xim’s post)
2. Download and install updates from Apple in sequence (3.1, 3.2, 3.3, …) to the latest one. Apple Download
3. Reboot and everything should be working. I had to manually add program files\boot camp\bootcamp.exe to my startup folder as it wasn’t starting at login for the feature keys.
Note: Depending on the version of the keyboard, it may not be necessary to update all the way to the latest version.