Microsoft allowed users to Download window 7 ISO image of operating system by Providing Product key, Which then can be used to crate bootable USB to install windows.
It is also possible to Download window 8.1 ISO from Microsoft without product key
Download window 7 ISO image
If you’re user of Windows 7 and looking to download Windows 7 ISO image by product key. You required product key of your Windows 7 edition located on the box of Windows 7 or on the back of your laptop to legally download the ISO image of Windows 7 from Microsoft.
Steps to Download window 7:Download Windows 7 ISO image, Go to Windows 7 Software Recovery webpage, enter your 25 character product key . Click verify
Note: if Windows 7 came pre-installed with your new PC, you can’t download the ISO from the above link.