The Upgrade is the same installation as the full version, only the key is different.
You can get the cleanest install possible with an Upgrade version, booting from the DVD, selecting Custom install, and using Advanced drive tools to format the hard drive. Clean Install with a Upgrade Windows 7 Version
The DVD scans the hard drive when it is starting up and sees XP/Vista/RC and allows the Upgrade key.
You can even do an Upgrade to a new or pre-formatted HD, using workarounds that are widely available
and even given out by MS. This is a courtesy because 6 million beta testers mostly abandoned their old XP/Vista when they saw how good Windows 7 is.