You won't be able to activate Windows 7 without a legally obtained product key.
Click on Start and then Control Panel.
Tip: In a hurry? Type system in the search box after clicking Start. Choose System under the Control Panel heading from the list of results and then skip to Step 4.
Click on the System and Security link.
Note: If you're viewing the Small icons or Large icons view of Control Panel, you won't see this link. Simply double-click on the System icon and proceed to Step 4.
Click on the System link.
In the Windows activation area of the System window, you'll see the status of your Windows 7 activation and your Product ID number.
Note: The Product ID is not the same as your product key. To display your product key, see the guide on how to find the Windows 7 product key code.
Next to the Product ID, you should see a Change product key link. Click on this link to start the process of changing your Windows 7 product key.
When the Windows Activation window appears, enter your new product key in the Product Key: text box.
Click Next to continue.
Wait on the Activating Windows... message until the progress bar is complete. Windows 7 is communicating with Microsoft to make sure that your product key is valid and to reactivate Windows 7.
The Activation was successful message will appear after your product key has been validated and Windows 7 has been activated.
That's all there is to it! Your Windows 7 product key has been changed.
Click Close to close this window. You can now also close the Control Panel System window.
Not a Windows 7 user? See How Do I Change My Windows Product Key? for specific instructions for your version of Windows.