Windows 7 Professional
1. Select the Start button, right-click Computer, select Properties, and then select Activate Windows now.
2. If Windows detects an internet connection, select Activate Windows online now. You might be asked for an admin password or to confirm your choice.
3. Enter your Windows 7 activation key when prompted, select Next. Activation will start. This might take a few minutes.
4. Once done, it will say Activation was successful, click Close. At this point, you can go back to Properties and scroll down to Windows activation, it should now say Windows is activated.
If you prefer clean installation, please refer to How to perform Windows 7 Clean Installation.
Note: In case you need a Windows 7 ISO, please proceed to this link. Here’s an alternative link.