Windows Defender.
It's built into windows 10 and I believe 7 (and if not it can be downloaded for it for free).
After years of using AVG, AVAST etc etc in reality they are not much better and cause many additional issues.
I would suggest however in addition to Windows Defender to add Ublock Origin to chrome and use safe browsing habbits (don't click banner adds saying "You have won as our one millionth visitor" that you see every time you go to a site etc etc).
Between those that will have you 95% covered versus AVG or AVAST which may have you 97% covered, but impact your system performance, create issues connecting to software and services, displaying false positives etc etc.
The paid systems may take you from 97 to 98 but cause even further issues.
I once spent an entire day trying to diagnose a computer for a client, our software on their machine was not connecting to our software on their server and I couldn't figure out why.
In the end It turned out to be due to Nortan Internet security, they had that installed previously and un installed it however it had left parts of its "Firewall" installed in windows with no information or indication that it was there or still doing anything.
That is just one issue of many, McAffee is just a resource hog and really will slow your machine down with "Bloatware"
So yeah, don't bother with after market AV , just use the built into windows one, install UBlock ORigin for chrome which will help against malicious banners and bad adverts and you will have all but a couple percent of your bases covered.
All information here is my own opinion but I am a software developer by trade so take that for what it's worth
