My understanding is to use Windows 7 upgrade media you must install and activate Windows Vista or Windows XP. In the case of upgrading from Windows XP a "clean install" is performed, but in reading about this, it sounds like this may just be overwriting the Windows XP installation, as it talks about the old Windows being moved to a Windows.old folder. It sounds like the same kind of thing you get when you install Windows over itself, which if you are familiar with that you know it leaves all kinds of junk from the previous installation.
This leads me to believe that this won't be a true clean install, in terms of it being identical to performing a non-upgrade install. What is this Windows.old folder, and what other "junk" will be left behind from the Windows XP install?