You don't have to change anything in the system registry to download the SP3 RC1 seen at the link there or for any other MS update. For a fresh installation or reinstall of Windows you first have to see the Genuine Advantage tool installed following the activation of Windows before being allowed to download anything from the link there or at the update site.
The Genuine Advantage is the latest MS method for fighting software piracy.
http://www.microsoft.com/genuine/Pro...d-4e0b19485092