The Windows Experience Index for your system could not be computed

The Windows Experience Index measures the capability of your computer’s hardware and software configuration and expresses this as a base score. A higher base score generally means that your computer will perform better and faster. I find this utility useful when performing certain tasks that require certain resources. For example, I expect to be disappointed when running graphics-intensive games while my current Gaming graphics sub-score is less than 3.5!
Each component (Processor, RAM, Graphics, Gaming graphics, and Primary hard disk) gets an individual sub-score and the base score is determined by the lowest score. Therefore, the base score is the lowest value amongst the sub-scores! When you upgrade the hardware, you may need to re-run the test as to find whether your new hardware performs better!

Windows 7 N and Windows 7 KN editions include the same functionality as Windows 7. However, these editions of Windows 7 do not include Windows Media Player or other Windows Media-related technologies. As such, the Windows Experience Index requires multi-media support to be able to run successfully and hence, fails the test if the Windows Media Feature Pack is not installed. For more details read MS article 968211

Users of the N editions need to download The Media Feature Pack for Windows 7 N and Windows 7 KN -  The download verifies that you are running a genuine Microsoft Windows. Then, follow the instructions in the Windows Update knowledge base 968211. When done, you should be able to run the Windows Experience assessment without any problems :)