I like Avast. In addition to the expected scan for malware, Avast offers three other security-related scans. And it supports real-time protection which is helpful when I'm mistakenly clicked a dangerous website.
I've used Avast Free ever since it became available and have been quite pleased with it. I have, however, do have a problem with it and W10 10240 as follows: if I right click the Avast icon in the system tray and disable the shields, it's "game over". Instead of the icon displaying a "x" to indicate it's disabled, nothing changes and, except for the mouse cursor, my system is frozen. No response to left click, right click, double clicks and the keyboard is dead. I've let it sit for 10 minutes to no avail. Ultimately I have to hold the power button until the computer (Dell M6700) shuts off. Don't know if it's just me or not. At least I've never lost anything. (And yes, there is a reason for my disabling the shields and no, I'm not stating why).
Avast! Avast (NEVER INSTALL ANY TOOLS!) works great on old/slow AND new machines! After MS dropped the ball on MSE..... Avast! It works AND it's free!
I have used Microsoft Security Essentials or Windows Defender for years with great results. They have kept my PC virus free. It also depends on your actions using your PC. Our internet provider offers Kaspersky 2016 for free, but I don't use that, slows my system down too much. Here a couple of recent news stories about anti-virus companies that are very interesting. Google the story titles for links to the articles. "Security wares like Kaspersky AV can make you more vulnerable to attacks" - Sep 23, 2015 - by ars technica "AVG Antivirus Plans to Collect & Sell Your Personal Data to Advertisers" - Sep 19, 2015 - by The Hacker News