Microsoft Windows Vista and Windows 7 uses a module (Windows System Assessment Tool) to measures various performance characteristics and capabilities of the hardware it is running on and reports them as a Windows Experience Index (WEI) score.

This rating score is a quick way to view and understand the hardware power of the machine to compare with a software (game or video edit app) which may put up a certain minimum system rating requirement to understand the possibility of operation.
Though its strongly recommended on not to touch the system generated report card, but some advance users may like to play with their index score to modify the numbers without altering the hardware.
How to edit or change the Windows 7 Experience Index Rating ?

Windows 7 Rating changer is a small and free app, which would easily do the job for any non technical PC users.