I use the built-in Defender on Windows 10, Microsoft Security Essentials on my Windows 7 installs. However for the past year I've installed Panda Cloud free on 7 installs were the user is prone to infection because it is a bit more aggressive while still having a very low performance hit.
I always install Malwarebytes and SuperAntiSpyware scanners for when users suspect they are infected, or to regularly scan every 2-3 months.
If someone is regularly infected, I install the MB trial and urge them to buy it after 30 day trial. None of these have ever been infected again. Most were getting caught up by fake AV scanner scams which come in on a webpage popup and look official enough that users click on them and infect themselves. These are as deadly as their cousins, Registry Cleaner ads, which are always scams and show up here ruining Win7 about once per week. Use CCleaner Reg Cleaner only.
Smart browsing is always important. Be aware of what Add-Ons or Extensions are operating on your browsers. I dont' allow anything except Flash, Silverlight, a reader helper and my standard Firefox translator and download manager.
Any installed Google program or staying signed into their sites is pernicious spyware which follows you every where you go pasting ads. For this reason I dont' install any google (Chrome, etc) or stay signed into their sites (YouTube). I also avoid interest ads for the most part by regularly running
Opt Out of Network Advertising.