1. Click Start and type CMD in the Start Search menu
2. Right Click and choose Run as administrator
3. To remove any existing product key (in case you used a trial key), enter and run the command slmgr.vbs /upk .
4. Clear the product key from registry by running slmgr.vbs /cpky
5. To enter your new product key, use the command: slmgr.vbs /ipk xxxxx-xxxxx-xxxxx-xxxxx where the x’s are your actual product key.
6. Lastly, activate Windows by entering the command slmgr.vbs /ato
7. Windows is now activated.

to check windows activation status type:

slmgr /xpr and hit enter