system
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I have to say that I have had Windows Defender running for a long time (even back to the days when I used MSE) and never had a problem with it conflicting with any AV program I have run in that time. However, I also have to say that it has never detected any problems either, so I guess now it the time to kick it to the curb. 
I will just have to remember to run MBAM more frequently. (Never mind, bought the Pro version.
)