Windows Defender in Windows 7 was not a true anti-virus package, and was superseded by Microsoft Security Essentials. When MSE is installed in Windows 7 I believe the old Windows Defender was automatically disabled. From Windows 8, an enhanced version of MSE was released that adopted the old Windows Defender name, but this time it is a true anti-virus package.
You should never have more than one anti-virus program performing real time scanning on a computer. Multiple packages can interfere with each other and actually reduce protection to a level below any of the packages running in isolation. If you have Windows 7 running Windows Defender, then that is the old version that was not a true anti-virus. It would be very unwise to rely on that to provide protection.
MSE as used on Windows 7 and the new Windows Defender in Windows 8 and later versions, although a true anti-virus packages, are not as effective at protecting Windows systems as Norton.
When Norton is installed on Windows 8 or later, then it should automatically disable the real time part of Windows Defender. I do not know whether this mechanism also applies to MSE and the original old type of Windows Defender.
Since you say you are running Windows 7, I am surprised that you did not mention Microsoft Security Essentials. Does this mean that your system is lacking any security updates and service packs?
I hope this helps.