Microsofts Genuine Advantage program will check whether you have a registered copy of Windows.
This is a bit of a liberty – don’t they trust me? I have a fully licenced version of XP Pro on this machine.
OK I don’t mind performing this once to download a particular piece of software. However once I have demonstrated to Microsoft that my machine is genuine that should be it. However Microsoft have a different opinion. Every time that you needto install software that is protected by WGA you need to go through the same hoops. How can my machine that is already a Genuine Windows machine stop being a genuine windows machine? Can they decide that it has been revoked?
Developer Rants 