To activate your Windows 7 edition, go to Control Panel > System and Security > System, then go to the bottom of the page and click Change Product Key.
If your product key is not accepted directly, or marked as invalid, you must go through the telephone procedure.
To do this, after entering your product key, click “Call Microsoft Activation Center”
Choose your country from the list and click Next.
Finally, call the number indicated on the window, then enter the numbers block by block.
Once the numbers are correctly entered, the voice server will return a list of digits to indicate below. Click Next, and then complete the activation.
Please note that you can do this step from a smartphone by calling from it directly, which will automatically send you a link to a mobile web page to complete the activation.