If there's anyone out there still using Windows or who otherwise needs an antivirus, just use ClamAV. It's free and open-source, does a pretty good job of finding viruses, can be run as a daemon, and is extensible.
Want a different index? Fangfrisch. Want a GUI? ClamTk. Using Windows? ClamWin.
I honestly find it so strange that it took me around eight years to discover that this was a thing. I was forced to use McAfee, Norton, Kaspersky, BitDefender, and all of the worst AV software on the market before I eventually switched to Linux.