According to Microsoft, Windows can be activated over the Internet by clicking "Start" then "Computer" and then "Properties." Click "Activate Windows Now" and then "Activate Windows online now," then enter your key. To activate via phone, click "Start," then "Computer," then "Properties" and "Show me other ways to activate."
According to HP.com, each version of Windows has slightly different activation procedures. An alternative way of finding the activation menu for your version of Windows is by pressing the Windows key on your keyboard, then the "S" key, then typing "Activate Windows." This brings up the windows activation menu options for your version of Windows.
To prevent software piracy, Microsoft supports a Digital Rights Management Scheme called Windows Genuine Advantage. WGA, among other things, requires that users enter a valid product key code within 30 days of installing the Windows Software, or the functionality of the software can be restricted by the DRM software. In addition to restricting access to Windows system components, WGA prevents users from installing security related updates. Activation notifies Microsoft that the copy of windows is installed on a computer, transmits the hardware specifications of the computer and deactivates the DRM clock, which locks down the software.