For years I used MS stuff -- Defender, then Security Essentials. Several weeks ago, out of the blue, it started flagging known-safe stuff I'd had unchanged on my system for years. I couldn't get it to ignore. So I uninstalled MSSE.
I liked the Avast reviews, so I tried it. Too many problems: Frequently the UI failed to load; ALWAYS I had to manually start real-time protection; had to reboot system after updating. Uninstalled it about an hour ago.
I want something with low incidence of false positives, low impact on system performance, and generally convenient.