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

Either shell out the extra cash for the full version or be prepared for the extended process of installing XP, and then Vista. Of course there is always the OEM road, but remember that such a copy is limited to the computer it is first activated on.

Source: DailyTech via Neowin