Defender is included as part of Security Essentials. If you have Essentials, Defender should not turned on.
Most of us in the know use Microsoft Security Essentials and Malwarebytes. Use the free Malwarebytes, run a weekly scan and you will not have problems.
In addition, I use Eset on Line for a weekly scan.
Now most members forget the most important anti malware and it is named Josh. You cannot find it on line, but you know where to find it. Use common sense, dont go to websites that you are doubtful about and most of all if you really want to go to a certain site for reasons that are not legit or proper, stay away. That is where most of the problems are found.
You can also use WOT, it warns of less than reputable sites. Not for only malware etc, but for potential scams. http://www.mywot.com/