There is a well known Windows Vista Upgrade secret in the IT community. In this article I talk about it and the possible legal implications! Please take this article "as-is" I provide no warranty of any kind by providing this information. Well to be honest it is no big secret as you can find this information anywhere online and many Microsoft staff must also know about it as they created this process! The "secret" is that the setup program (setup.exe) in Windows Vistas upgrade version will accept an installed copy of XP, W2K, or an un-activated copy of Vista itself as evidence of a previous installation.
...and it is this last option that enables you to perform a "clean install" of an upgrade version of Windows Vista to any formatted or unformatted hard drive! Basically you are installing Windows Vista twice to take advantage of this trick.
Is This Windows Vista Upgrade Secret Legal? Probably not. I am no legal expert but I am guessing this will violate some part of the Windows Vista EULA (End User Licence Agreement.) BUT Microsoft have created this process.... It is not something you have to try and crack in their software or run a third party application to perform this process... Their development team created it. Which kind of begs the question WHY?
Link: Undocumented Upgrade Option @ Instant Vista