The Windows 7 product key is used to activate a copy of Windows 7 for use on a specific computer. Each product key is unique, and activation is necessary to obtain access to all of the Windows 7 features.
The activation process pairs the product key with the computer that Windows 7 is installed onto. Activation must occur within 30 days of installation to keep all of the features of the software operating. If the owner of the computer waits more than 30 days to use the product key, or if the product key has been used for a different computer or is counterfeit, then the system is deemed non-genuine. At this point, the product key must be used to return it to a genuine state, or counterfeit keys should be reported and a genuine product key purchased.
Keeping the product key from a Windows 7 install is important, as reactivation is sometimes needed at a later date. Reactivation is required when the computer that Windows 7 is installed onto is changed. For example, the hard drive and the memory may be upgraded at the same time. Reactivation is also needed when Windows 7 is reinstalled onto the system for any reason.