Microsoft made some changes to the upgrade versions of Windows Vista. In the past you only needed to insert the disk of your previous Windows version but now you'll actually need to have the previous OS installed on your computer:
For example, when performing a clean install of Windows XP Professional using an upgrade CD, users would run through the normal setup routine until prompted to insert a previous version of Windows. A user could pop in a Windows 98 or Windows 2000 CD for upgrade compliance and then the setup routine would move along as usual.
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.
PC enthusiasts who want to start with a clean HDD will need to:
1. Install Windows XP
2. Activate Windows XP through Microsoft
3. Upgrade to Windows Vista from within Windows XP