Users can do a "clean" installation of Windows 7 using an upgrade licence to save $80-$100 over the price of a "full" edition, a popular blog reported today.

According to Paul Thurrott, who writes the Supersite for Windows blog, Microsoft's upgrade media and product keys can be used to do a full installation of Windows 7 on PCs that do not have an earlier version of the OS.

Microsoft's Windows 7 upgrades are designed to do "in-place" or "custom" installs on PCs running Windows XP, Windows Vista or one of the preview editions of Windows 7. But Thurrott said that a few simple steps lets users install upgrade editions, which are considerably cheaper than the full versions designed for fresh installations.

Windows 7 Home Premium, for example, costs $119.99 (£79.99 on the Microsoft EMEA website) as an upgrade, but $199.99 (£149.99) for the full edition, a difference of $80 (£70). The full versions of Windows 7 Professional and Windows 7 Ultimate, however, cost $100 more than the corresponding upgrades. (In the UK, Windows 7 Professional costs £219.99 while Windows 7 Professional Upgrade is £189.99, a difference of £30).

Nearly three years ago, Thurrott showed users a similar trick with the then just-released Vista; that technique, however, required users to install Vista twice.

Today, Thurrott spelled out how to conduct a clean install using a Windows 7 upgrade licence. The process requires users to install but not activate Windows 7 with the accompanying product key. After the installation is completed, users must make a minor change to the Windows registry, use the Windows "rearm" command, then reboot.