Also read that you can install a working vista installation using only the update Vista DVD if you install vista twice over itself on a clean partition.
details here:
http://gizmodo.com/gadgets/pcs/how-to-install-a-vista-upgrade-on-a-blank-hard-drive-232770.php
http://www.theregister.co.uk/2007/02/12/cheap_vista_for_everyone/
While this appears to be a mistake by Microsoft, the company intentionally made EULA-violating upgrades very easy for XP too. You can install an XP upgrade to a blank hard drive, after briefly inserting a Windows 98 CD to "prove" that you are upgrading. It is not necessary to provide a Win98 CD key, all you need is a real or copied Win98 CD.