From what I have read the win 7 upgrade can be used as a full install. You are only supposed to use it if you are upgrading a qualifying version of windows. If I'm retiring/recycling my old XP machine and building a new one would it be legal for me to use the win 7 upgrade on my computer since I technically have a prior version? How would it be any different then slowly updating my old computer with all new parts and then putting it on with a fresh install?