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Computerworld - Microsoft today confirmed that users can apply a workaround trick to do a clean install of Windows 7 on a blank hard drive as long as they toe the licensing line.

In a blog post earlier this week, Eric Ligman, who works in Microsoft's worldwide partner group, took exception to stories that showed people how to use the less-expensive Windows 7 upgrade editions to install the new operating system on blank drives. Computerworld covered the upgrade install trick -- first reported by noted Windows blogger Paul Thurrott -- last Friday.

"Over the past several days there have been various posts, etc. across a variety of social media engines stating that some 'hack' (be it a person or a procedure) shows that a Windows 7 Upgrade disc can perform a 'clean' installation of Windows 7 on a blank drive from a technical perspective," said Ligman.

"They often forgot to mention a very basic, yet very important piece of information," said Ligman about Thurrott's blog post and the resulting reports by others. "'Technically possible' does not always mean legal," Ligman said.

In order to use upgrade media to install Windows 7 on a blank hard drive, users must abide by the operating system's EULA, or end-user licensing agreement (download PDF). "To use upgrade software, you must first be licensed for the software that is eligible for the upgrade. After you upgrade, you may no longer use the software you upgraded from," the EULA states.

That, said Ligman, means users must either have a "full" retail license of Windows XP or Vista, or assuming the Windows 7 upgrade is applied to an existing PC, that the upgrade is done on that same machine, which has a so-called "OEM" license attached to it.

"There are many, many, many, many of you out there that already own Windows licenses that qualify for the Windows 7 Upgrade, so this is a non-issue for you," said Ligman. "For you, since you have the previous version FULL Windows license and qualify for the Windows 7 Upgrade, you have the rights to do a 'clean' install."

On PCs purchased with Windows XP or Vista preinstalled by the computer maker -- which slap an "OEM" license of Windows on the machine -- users can install a Windows 7 upgrade edition on that system's blank hard drive, but on no other, Ligman added.

"An OEM license is a full license," Ligman wrote in a comment to this blog post, answering a user's question. "So an OEM + an upgrade gets you the upgraded version."

A Microsoft spokeswoman today confirmed Ligman's account of when it's permissible to use upgrade media -- which costs up to $100 less than the same version's "full" edition -- to install Windows 7 on a blank drive. "You can always do a clean install if you're upgrading, so long as you're upgrading a machine that's already running genuine Windows XP or Windows Vista," she said in an instant today.