QUOTE
Unlike with the Windows XP Upgrade CD, with Windows Vista users are forced to have a valid install of Windows XP Home or Professional on their machines before upgrading with a Vista Upgrade DVD. This typically isn't a big deal for the majority of users on the first install of Vista; but think about the necessary steps if you already have Vista installed or no operating system on the computer:

1. Install a genuine copy of Windows XP Home/Professional
2. Activate Windows XP through Microsoft
3. Upgrade to Windows Vista from within Windows XP