Upgrading a user to Windows Vista from Windows 2000 | User Support | TechRepublic.com:

For this option, when you try to activate the software, youโ€™ll be told, The Software Licensing Service determined that this specific product key can only be used for upgrading, not for a clean install. At this point, all youโ€™ll have to do is install Vista again, but this time youโ€™ll be upgrading to Vista from Vista. That sounds strange, I know, but itโ€™s true. When you install Vista a second time, youโ€™ll want to do it from the new installation of Vista, not by booting from the DVD. You do want to enter the product key the second time around, and you do want to choose the upgrade option.

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