I figured it out.
There is a BootCamp.msi on the CD that came with the iMac. This did it. In the end, it seemed as though AppleKeyboardInstaller.exe wasn't even necessary.
Even afterwards, Device Manager still shows it as a "HID Keyboard Device" but it is working.
The AppleKPro did not work. It could be that it's not for this keyboard, or it could be that it's not for Vista. It simply did nothing when I clicked "agree" to the license agreement screen. Also it is payware and I was hoping for a free solution, which I found. (Well, the iMac was $1799, but I didn't have to pay for extra software! :) )