A few of our customers who have purchased the Upgrade versions of Windows Vista cannot perform clean install. You are forced to install their previous OS and then upgrade to Vista.
What a time waster.
The problem is because Windows Vista does not check for upgrade compliance. Therefore, you cannot use an upgrade key to perform a clean installation of Windows Vista.
Looks like Windows 7 followed the same route.
If you purchased the upgrade, just stick the CD in and don’t format first.