To activate your Windows 7 Operating System, you must have a valid product key.
Go to Start button and enter Activate
in the Search TextBox.
You will see Activate Windows. Click on Activate Windows.
On next screen, you will see Activate Windows online now. Select this option.
You will go to next screen when you will have to enter your product key.
Go ahead and enter or copy and paste your product key in the Product Key text box.
Click Next button.
You will see Windows Activation dialog and the following message.
Allow it a few minutes.
After successful activation, you will see the following Activation was successful message and you are good to go.