It is a commonly-known fact that Microsoft allows would-be users of Windows to install and run the operating system on their machines for up 30 days without entering a product activation key. Things will be no different with its latest and greatest, Windows 7, but reports on Windows Secrets today reveal that it is possible to reset this trial period up to three times using a less-familiar command.
The procedure is really simple, too - just open a command prompt window, enter "slmgr -rearm" (minus the quotes) and reboot. The 'trick' will work with any version of Windows 7, from Basic to Ultimate, and has been confirmed to be completely legal by Microsoft. According to the company, the feature was also available in Vista and is aimed at corporate administrators who need to delay activation for whatever reason.
Retail customers should see no reason not to activate their copy of Windows 7. That said, if you happen to come across the release-to-manufacturing version, you should be able to use it legally and free for up to four months without entering an activation key.