I've been running it on Vista 64-bit with no problems for a few months now. Just have to make sure your UAC is disabled (i prefer it that way anyway) but I didn't have to make any tweaks or anything.

I logged on from my own computer to my dad's account to download viper (i'm a previous user) and then went back in to try and set up my seperate subscription. I was logged in as myself, but for some reason it upgraded my dads already premium membership again. Which, btw, is BS. How can you pay for 2 ...