Here's the "hack" that Microsoft provides:
You have met the requirement by having the OS needed for an upgrade.
http://www.microsoft.com/windows/products/windowsvista/buyorupgrade/upgradepaths.mspx
(For instance XP Pro to Vista Home Premium)
A 'clean-install' is required by some upgrade paths and MS failed to provide
a 'clean' method of doing this with their upgrade disk, but they did provide
a method.
The instructions MS provides are:
http://support.microsoft.com/kb/918884/en-us
But if the path you desire for upgrade is not available, the Upgrade button
is disabled. and you must select Custom Install. Unfortunately, this will
not work with Method 1 or Method 2 unless you fail to insert the Product Key
on the first installation. (Which they conveniently forget to tell you.)
In a nutshell:
This method erases ALL data from the selected drive!
Boot from the DVD and select Custom Install.
Format the drive and install Vista (version you bought) without entering
your Product Key.
When it is all done, while running Vista from the Hard Drive, open the
DVD and install again, this time selecting Upgrade. (Total time: about 1.5
hours on my machine.)
Don't bother installing software/updates until you complete both installs.