Personally, I wouldn't even install Steam in the Program Files folder.
I would recommend that as well, but I know little to nothing about Steam. Therefore, I would not know how to tell users how to go about moving it.
As for the UAC, you should Google it. You need to know what that is since that is an integral part of your OS. Well, maybe not "need," but that would probably be helpful information to have. I do not use Win7 or Vista, but that will hopefully change this summer.