Microsoft has quietly posted Knowledge Base Article # 930985 on their website, which states that “You cannot use an upgrade key to perform a clean installation of Windows Vista”

Towards the bottom of the article it states that this applies to:

In previous Windows versions, you were able to purchase a upgrade version, format the hard disk, and during a clean install you were prompted to insert the disk for your previous version to confirm you had a previous version, then the installation would proceed. You would end up with a clean installation of Windows without any leftover ‘crude’ from the old operating system. This was the preferred method.

Why? After years of using any operating system, ALL systems build up ‘gunk, junk and crude’ on them. It is just the nature of the beast. So a clean install normally eliminates this build up and normally provides for a smooth operating system transition.

So it appears that if you buy the upgrade edition of Vista, using one of the editions above, it will only install if during installation it sees a previous version of Windows, or more specifically, Windows XP or 2000 which is explained here.

FWIW - what is the difference between a Upgrade and Full versions? Nothing. Both are exactly the same animal. But now there appears to be some exceptions which are worth noting.

[tags]vista, upgrade, change, clean install, [/tags]