Microsoft has relaxed the Windows Vista EULA since 2006. But the modifications brought by the Redmond Company to the end user license agreement were not as extensive as you might think. Originally, Microsoft had stipulated in the Windows Vista EULA restrictions related to the installation process of the operating system. Following consumer feedback, Microsoft has then increased flexibility on behalf of the user.
However, License Terms restrictions for Windows Vista Home Basic, Home Premium and Ultimate are still in place. And they refer to the upgrade process of Vista. Microsoft wants no less of its users than to throw away their old copy of Windows once they upgrade to Vista.
“To use upgrade software, you must first be licensed for the software that is eligible for the upgrade. Upon upgrade, this agreement takes the place of the agreement for the software you upgraded from. After you upgrade, you may no longer use the software you upgraded from,” reads a fragment of the Microsoft Software License Agreement for the three Vista editions mentioned above.
Via the upgrade license of Windows Vista, users agree to no longer use their previous copies of Windows XP if they upgraded from XP, or Windows 2000 if that's the OS from which they moved to Vista.
It is not enough that Windows users can only deploy Vista if they have a valid older version of Windows in place, and they cannot perform a clean install with an upgrade key. Also, it is not enough that, in order to upgrade to 64-bit Vista bought via the Windows Marketplace, they must do it from a 64-bit edition of Windows XP. Now Microsoft is asking them to stop using the previous copy of Windows after upgrading to Vista.