Make sure you disable the driver integrity/signature check (F8 at boot-time) before you attempt to install the drivers, and also every single time you boot after that (unfortunately). bcdedit doesn't seem to work anymore, so you have to disable the check at every boot as long as you want to use unsigned drivers. I finally got 5.1 sound working on my Audigy2 in RC1 x64, but it was a pain in the ass to say the least...