Even from the command line?
Click Start, type: cmd
Right click CMD click Run as administrator
At the command prompt, type the following commands:
slmgr.vbs -ipk xxxx-xxxx-xxxx-xxxx (allows you to replace the current product key with the specified)
xxxx-xxxx-xxxx-xxxx - represents your product key
Hit Enter on your keyboard
Exit the command prompt
Restart your computer
If that does not work, I would recommend repairing your installation: