I've been running windows 7 rc x64 on my home built system ever since I built it. A few days ago I ordered the $49.99 windows 7 from best buy, but now I hear that that it's only an upgrade and you had to have vista or xp for it to work. Is this true and it will not work on my computer, or will it? If it doesn't, do you think I can exchange it with best buy when I finally get it?