I'm going to revive this excellent thread to say that it still works! I've just used the BootCamp 5.1 files provided by nickjf20 (on page 6)
I do have one question, if someone still reads this old thread: I had an Overlay for Volume etc working previously, and it wasn't either Autohotkey nor AppleWirelessKeyboard. It was some overlay that did very little, but it did it well. It added the capability to use the Fn-keys for media, and added the overlay, and it also added the Apple type "blop" sound when you changed the volume.
Any ideas what it is I'm looking for?
After some Googling, I found it.
First I found the thread from 2012 on Lifehacker that proposed AppleWirelessKeyboard. And reading through the comments, I found the guy who had created the tool I'm proposing (and that I've been using). Problem: He didn't name it, and the link was dead. Luckily, he had named it; he said: "To enable the volume keys: -Install 3RVX".
So back to Google, and I Googled for 3RVX. And lo and behold! He's still developping that small excellent tool!
If you're a user of any Apple Keyboard on Windows you want this! Go grab it!
And huge regards to m4xim, Bergdoktor, nickjf20 and the rest in this thread, that have all made it possible for me to use my wireless keyboard since 2010!