Microsoft allowed users to Download window 7 ISO image of operating system by Providing http://littlebitjohnny.blogspot.com /2013/09/product-key-for-windows-7-ultimate-32.html" target="_blank">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
Steps to Download window 7:Download Windows 7 ISO image, Go to Windows 7 Software Recovery webpage, enter your 25 character http://littlebitjohnny.blogspot.com /2014/04/windows-7-ultimate-product-key.html" target="_blank">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.