In my experience, you can't do that with an "upgrade" disk.
I have no experience with Vista or Win7 Upgrade versions, but I do happen to know for a fact that you could perform
a completely clean install using a 95, 98, ME, 2000, or XP (Home) upgrade software. If it doesn't find an old Windows
installation on your hard drive, it asks you to insert the disk during installation so it can verify that you do, in fact, own
it.
Now like I said... it's been ages since I've done this, and I don't recall every trying with XP PRO... and definitely never
with Vista or newer... but still.. I find it hard to believe that Windows would refuse a clean install, even with an 'upgrade'
disk.
