The workaround that enables users to perform clean installations of Windows Vista in the absence of older versions of Windows breaks the Microsoft Software License Terms. The Microsoft End User License Agreement for Windows Vista Home Basic, Home Premium and Ultimate clearly states that - in order to deploy the upgrade version - the user must first own an original license of the software.

"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 an excerpt of the EULA.

Microsoft has limited upgrades to Windows Vista only to users that have already installed a genuine copy of Windows. The limitation is due to the fact that Microsoft has not designed Windows Vista to check for upgrade compliance. This is why Vista must be installed on top of an older and genuine copy of Windows.

However, according to the EULA, you needn't deploy Vista on a machine already running a prior version of Windows. You can do a clean install of Vista, provided you own a license for the "software that is eligible for the upgrade."

Microsoft does not expect a great deal of users to break the License Terms they must agree to when installing Windows Vista, although by acquiring the upgrade versions of Vista as opposite to the complete licenses you can shave a hefty amount off the full price. Additionally, Microsoft does not plan to patch the workaround that permits clean installation of Vista via upgrade keys.