A loophole in Windows Vista allows users to perform a clean installation of the operating system from an upgrade DVD.

A clean installation generally performs better than an update. Upgrade versions of the operating system also are roughly 35 per cent cheaper than the full versions.

Microsoft provided so-called upgrade keys with previous operating systems that allowed the software to determine whether a user was eligible for a discounted upgrade. 

The process required users to insert the installation CD for their old operating system at some point during the installation.

But Windows Vista does not verify upgrade compliance in this way. Instead it blocks clean installations from an upgrade disk and requires that the set-up file for the upgrade is executed from inside an existing operating system.

Microsoft support, however, is providing information on having a clean installation on a used system, IT author Paul Thurrott claimed. 

The company said that delaying ordering the product key will result in a clean installation.

After the installation is complete, users have to run the installation file again from within Windows Vista, enter the registration key and essentially upgrade the Vista installation with another Vista installation.

Microsoft did not immediately respond to a request for comment.