Microsoft Security Essentials and Windows Defender are NOT the same product, at least in Windows 7 and earlier.
Defender was mostly about malware, MSE was mostly about viruses.
In Windows 7 and earlier I think most people had Defender (but maybe did not know it) but MSE was an optional download.
So if running Windows 7 or earleir you can install MSE even if you have Defender installed.
But in Windows 8 then Defender and MSE have merged and it is now called just Windows Defender (which includes MSE).
And I believe it now installs as your default security software when Windows 8 is installed.
You dont need MSE in Windows 8 as it is part of Defender.