In Windows 7, Windows Defender and Microsoft Security Essentials are 2 separate programs.
Users can only have one or the other, not both. It is because MSE will shut down Windows Defender once it is installed.
In Windows 8/8.1, Microsoft Security Essentials is part of the operating system and it is called as Windows Defender.
Users cannot install MSE as separate program as it is already part of the operating system.