I read on the internet, and saw on tv, heard on the radio, and on about every other billboard, and pretty much anywhere else I look that a mask is totally effective at stopping viruses. I guess on a computer you're supposed to put it on the exhaust side of your fan. After all, it's not to protect your computer but to show other computers around that you care about limiting the spread of dangerous viruses.

Oh wait did you mean something else? Sorry, it's the first thing I thought of so I had to get that joke in there. Serious answer I don't really run AV but if I were going to I'd pay up for Kaspersky. I don't trust free software that isn't open source anymore. Particularly not something I'm going to leave running and likely automatically update. The other big names McAfee and Symantec the last time I used both of them it felt like I was being asked to pay them for the privilege to spy on me. If you're set on wanting something free for Windows that isn't the built in defender, I used to run AVG on a big network a decade ago and it wasn't bad. It was dirt cheap for enterprise, but is free for personal use. Web-based malware was rampant back then and it was surprising how much stuff it caught. Also let a lot through, but that was user stupidity for installing malware intentionally not knowing it was malware, not really anti-virus stuff.