Right-click on the Computer icon on your desktop, and then click Properties, or use the Windows and Pause hotkey to open the System window. Alternatively, if you’re on Windows 8.1, you can right-click the Start button, and then click the System option to open the same window.

Once the System window is launched, scroll down to see the Windows activation section. Here, click on the link labelled ‘Change product key’ to see a dialog saying:  ‘Enter a product key screen’ or ‘Windows Activation dialog’ in Windows 8.1 and 8 respectively.  In this dialog, you need to enter your 25-character product key, and then press the Enter key, or click the Activate button to connect to the activation server and activate your product.