OEM keys work fine on non OEM disks. They are the same.
Go here and download the OFFICIAL iso that matches your license.
www.w7forums.com/threads/official-windows-7-sp1-iso-image-downloads.12325/
Use the windows USB tool or burn to DVD. Do a clean install and don't enter your license key. its not required yet. Once install go to activate online, enter your key and it should activate with no problem.
This ASSUMES you really have a valid license and key. Obviously if you don't this won't work and it will just end up saying its not genuine.