I was same. I thought Defender is enough to protect my PC.
however
I got virus two times and Windows defender won't able to deduct it even with scan.(last time if it was a Thor icon which is duplicating itself in every drive and every folder)
Then I installed Bitdender free version which finally able to delete that.
I am not talking about paid vs free. But Defender is the worst possible anti-virus out there which won't deduct most of virus. It failed me 2 times.
I think there are lot of good free anti-virus are there which don't effect system performance and not take much space in hard disk..
a little bit of extra security won't hurt it or is it?
Most of users forgetting this but Defender isn't enough
http://www.windowscentral.com/do-you-need-antivirus-program-your-windows-pc-2015
I think most of users here forget this but this is very educating article. Please read it again.