A retail license lets you install Windows on one computer.
If you try to install it twice, Windows activation can tell (so long the computers have a connection to the Internet) and will call foul.
If you do intend to try this stuff out for yourself, I recommend extreme caution.
My hunt for utilities that bypass Windows 7 activation technologies led me to some very seedy corners of the Internet.
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Today, Microsoft has online activation servers that must validate your installation key, and your installation key is often tied to your computer.(I documented this process in How to Clean Install Windows Vista with Upgrade Media.) When it comes to performing a clean install of Windows 7 using Upgrade media, there's no simple answer.For some people, it just works, and I'll provide information here so you can help ensure it just works for you, every time.For more information on why activation is necessary, see the links at the end of this article.For more information on how to successfully activate Windows, see Activating Windows when swapping between Boot Camp and a virtual machine in VMware Fusion (1003426).The activation process in Windows Vista and Windows 7 is different from the Windows XP activation process.