Windows Defender is the equivalent of using a discarded party balloon as a condom. It's better than absolutely nothing, but a complete joke if you're looking for protection. Every independent analysis of A/V software ranks it dead last...by a wide margin.
My recollection is that while it can be run with other A/V in Windows 7, in Windows 8 installing anything else turns it off...which is virtually always a good thing...well unless you opted for Symantec ....