The upgrade edition of the operating system (Vista) could be installed on a "clean" hard drive. For whatever reason, Vista had been programmed to accept itself as a "qualifying product." This eliminated any need for users to purchase the full edition of Vista or to upgrade Vista only over an older instance of Windows.
Ironically, the original release of Vista's upgrade edition was disappointing to many consumers. They'd been told by Microsoft that the Vista upgrade process would no longer accept the insertion of a disc containing an older version of Windows as proof that Vista was upgrading over a qualifying product.
Instead, users heard from Microsoft that the Vista upgrade procedure must be launched while a copy of Windows 2000 or XP was actually running. The upgrade trick that Vista developers included, however, renders that requirement moot. A Vista upgrade disc will install and activate properly even on a blank hard drive that has never previously been used.
Installing software from an original distribution disc to an empty hard drive, which is called a "clean install," is a best practice recommended by security organizations, such as NIST and US-CERT. Vista, unlike XP and previous Windows versions don’t make a clean install easy.
Customers that want to perform a "clean installation" of Windows Vista using an upgrade DVD can do so provided they have an older version of Windows already installed on their PC's hard drive.
(Performing a clean installation using only media from an older version of Windows is no longer possible, as Windows Vista does not include disc checking for old versions.)
Microsoft are also reminding customers who purchase retail upgrade versions of Windows Vista, that are not permitted to reinstall or reuse their old copy of Windows XP on an additional, separate device. This is part of the end user license agreement (EULA) the customer consents to by purchasing a retail upgrade version.
This seems to me to strike a fair "balance" for customers. Upgrade versions allow you purchase Windows Vista at significantly reduced prices after all.
If you purchase a FULL version of Windows Vista you DO however maintain the right to install your previous version of Windows on a new device of your choice.
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