Consider the following scenario. You purchase an upgrade license and an upgrade key for Windows Vista. You start the computer from the Windows Vista DVD, and then you try to perform a clean installation of Windows Vista by using the upgrade key that you purchased. In this scenario, you are prevented from continuing the installation of Windows Vista.

Note: In a clean installation, you completely delete the hard disk of the computer that you are upgrading. You do not install Windows Vista directly over the previous operating system.