Go to the website Mahmood and find the software called Windows 7 activator and download that software.
Now go to the location where you saved that software and double click that software's icon to run it. When software screen comes up now click Install/Remove 7 loader icon and install it. Restart the computer and it's done.
Now your Windows 7 has activated. You can check it in your my computer's window where you can see the Windows 7 genuine logo.