I know everyone has their favourites and loyalties when it comes to antivirus software but you can't beat Sophos Home IMHO. I had the brilliant professional version looking after all my servers when I was working and now Sophos have brought out a free home version that you can use on multiple and cross-platform PCs, plus apps for Android and Apple phones, etc. Sophos works unobtrusively in the background, without constant reminders to buy a different version and with no drain on performance like some of the bloated packages you pay for.
I'm probably teaching my Grandpa to suck eggs but for those who don't know, it's important to uninstall one before installing another. Having two antivirus programs on a device will cause no end of issues!