Microsoft has cut out this process for Windows Vista and forces users who buy an upgrade CD to actually have a valid install of Windows XP Home or Professional on their machines before upgrading.
For most users, this wouldn't be a problem. They more than likely have an existing copy of Windows XP installed and would have no problems upgrading to Windows Vista with an upgrade CD.
But for do-it-yourselfers who buy a Vista upgrade CD and think that they can easily perform a clean install whenever they feel free are going to run into the road block. In this case, the road block means that users wanting to perform a clean install with a Vista upgrade CD will have to:
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
So if you plan on saving money by using a Vista upgrade CD instead of purchasing a full copy, be aware that you?re have a few extra steps involved before booting to the Vista desktop for the first time. If you don?t want to deal with the hassle, you?re going to have to pony up for a full copy of Vista.