Note: I am aware this may seem similar to "Antivirus for Windows (to replace Avast Antivirus) meeting specific requirements"
I have been using Avast! Free Antivirus for quite some time on Windows 10, and it hasn't helped much. In fact, it made certain things much worse such as by removing some crucial device drivers, slowing down the computer by a ton (8% CPU usage idle on a fast processor), showing unwanted ads, and even taking down some of the core security features of Windows 10 such as the pre-existing firewall. By the way, this all happened when the trial period ended.