I had Vista 64bit on my first build with BF3 Demo and everything worked fine. The only problem I noticed was that you can't have two things installing at once, it would pause one install until the other one was finished. I upgraded to 7 now.
I've heard Windows 8 will be annoying for PC gamers because it's going to copy the game shortcuts in several different locations, little things like that.
Also, let's get back on topic