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Computerworld - Users can do a "clean" installation of Windows 7 using an upgrade license 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 as an upgrade, but $199.99 for the full edition, a difference of $80. The full versions of Windows 7 Professional and Windows 7 Ultimate, however, cost $100 more than the corresponding upgrades.

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 license. 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.

"When Windows 7 reboots, run the Activate Windows utility, type in your product key and activate Windows," Thurrott said. "Voila!"

The rearm command can also be used to run a copy of Windows 7 for up to 120 days without activation, a trick that Microsoft confirmed two months ago.

Microsoft allows users to install and run any version of Windows 7 for up to 30 days without requiring a product activation key, the 25-character alphanumeric string that proves the copy is legitimate. The rearm command can be used as many as three times at the end of each 30-day grace period to extend the activation-free ride for approximately four months.

Some users commenting on Thurrott's blog reported that they were able to do a clean install using a Windows 7 upgrade key without resorting to his registry hack and the rearm command. Thurrott, however, was skeptical.

"It certainly doesn't hurt to try this, but my guess is that there was a version of Windows on the hard drive that Setup detected, thus making the install and activation work properly," he said.

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