http://support.microsoft.com/default.aspx/kb/930985/en-us
Upgrade installation keys are blocked when you start from the Windows Vista
DVD
SYMPTOMS
You purchased an upgrade license and key for Windows Vista. When you start
from the DVD and then try to install Windows Vista by using this upgrade
key, you will be blocked from continuing.
CAUSE
This problem occurs because Windows Vista does not check upgrade compliance.
Therefore, you cannot use an upgrade key to perform a clean installation of
Windows Vista.
RESOLUTION
To resolve this problem, use one of the following methods.
Method 1
Start the installation from a compliant version of Windows, such as Windows
Vista, Microsoft Windows XP, or Microsoft Windows 2000. After you have
started the installation, you can select Custom at the installation choice
screen to perform a clean installation.
Method 2
Purchase a Full Product License. This license will let the installation
continue after you start from the Windows Vista DVD.
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