Update March 2020
I haven’t used any 3rd party Anti-virus since upgrading to Windows 8.0/8.1, with the exception of Malwarebytes pro (now home premium.) I’ve used Malwarebytes (MBAM) for many years as part of my “virus/malware removal tool kit and I’ve found that MBAM removes stuff that other (paid for) AV products fail to remove.
Windows Defender,(now called Windows Security) imo, does a great job and plays nicely with MBAM. The products basically do the same job, in a way, but do so differently, which means they don’t get in each others way. It seems to me that other 3rd party AV products actively disable any protection that Microsoft have put in place to enable their products to work but that might not be the case with all 3rd party AV products.
Microsoft have now taken charge of security and updates. IMO, Windows Security is all you need to defend against viruses, malware and other security issues. The new improved software firewall pretty much negates the need for any 3rd party firewall but the User Interface (UI) could be more friendly 🙂
I still recommend installing the free MBAM as an addition to Windows Security and running regular scans to make sure nothing is getting missed. If you install MBAM Home Premium, you should disable the “real time protection” from either MBAM or Windows Security.