To register your Windows operating system, activate it via the System window in the Control Panel if you’re using Windows 7, or use the Activate Windows functionality via the Change PC Settings screen in Windows 8. For Windows 10, enter the product key during installation, or use the Digital Entitlement option if you upgraded to Windows 10 from a genuine copy of Windows 8.1.
In Windows 7, open the Start menu, right-click on Computer, select the Properties option from the drop-down menu, and click on the Activate Windows Now link. If you have an active Internet connection, click on Activate Windows Online Now, type the product key that came with the Windows 7 installation disc, and follow the instructions from the activation wizard to complete the process. If you don’t have an Internet connection, click on Show Me Other Way to Activate, type the product key, click on Use the Automated Phone System, call one of the numbers, and follow the instructions from the automated system.
In Windows 8, press the "I" and the Windows keys at the same time to open the Charms bar, and click on Change PC settings. Open the Activate Windows tab, click on Enter Key, type the product key in the appropriate field, and enter your password if the User Account Control message appears. For a new installation of Windows 10, the operating system registers automatically once you enter the product key during the installation process.