Microsoft debuted Windows Experience Index (WEI) tool in Windows Vista to measure your PC components performance. As such, each key hardware device in the PC, including the microprocessor, RAM, graphics (for the UI and for more advanced tasks), and hard disk, is assessed and awarded a sub-score; and the lowest scoring component is used as the basis for the system's overall WEI score. In general, a higher-scoring PC will perform better, overall, than a lower-scoring PC. Likewise, a higher-scoring component on one PC likely performs better than a lower-scoring component on a second PC.
Microsoft provides the following information about interpreting a computers base score:
1.0 to 1.9 - Basic performance. Even a component with a 1.0 subscore meets the Windows 7 minimum requirements.
2.0 to 2.9 - Enhanced performance. Windows Aero may be available.
3.0 to 3.9 - Windows Aero will be enabled.
4.0 to 4.9 - Good performance with high-resolution displays and multiple displays.
5.0 to 5.9 - Capable of running high-end video games, 3D modeling applications, and high-end video editing applications.
6.0 to 6.9 - Supports DirectX 10 graphics with high frame rates at high resolutions.
7.0 to 7.9 - Highest performance level available. Reserved for systems with SSD hard drive, high-end graphics card, and multi-core (i.e. 8 or more) processor.
Source:→ Paul Thurrott