You won't be able to run an upgrade installation of Windows 7 over an install of XP, you'll have to do a format+clean install and provide an upgrade eligible license/product key.
What? I thought the promotion was offered to XP customers as well. I thought the only difference was that I had to do a clean install as oppose to vista? Isn't going from windows xp to windows 7 still an upgrade? Can somebody else please confirm this before I go and buy it?