Two antivirus programs on one system? Heresy, right?
Well, not if one is designed to play second fiddle. As it turns out, Avast, one of the best free antivirus programs, can do just that. Just download it, run the installer, and pick the second option–“Compatible install – install as the second line of defense.” Be sure to de-select the option to install Google Chrome and make it your default browser, unless you want that. Free programs often come with ridealongs.
Avast’s installer may give you a warning and ask you to continue. Say yes.
The idea behind having a secondary antivirus scanner is that no antivirus catches everything. But having a second engine available will help to catch anything that manages to get past the first engine. You’ll never get to 100%, but you can close the gap between what your primary antivirus catches and the ideal.
When you download something, I recommend right-clicking on it and scanning it with Avast before installing it. Presumably your primary engine scanned it as your browser wrote it to disk, but scanning it with your other antivirus engine won’t hurt. You’ll see that option in your right-click menu as well.
Nothing stops you from installing ClamWin as a third line of defense, since it’s only a scanner, but I’m not confident you gain much by doing it, since ClamWin’s success rate is less than 60%. I can’t prove it, but with a success rate that low, I would expect Avast to catch everything ClamWin would have caught.
To get true defense in depth, you need to schedule scans.
I found instructions here, but haven’t figured out how to make it work (yet) with the current version. Scheduling scans is easy, but non-obvious. Thanks to my buddy Tom for pointing out the settings hidden in plain sight.
I do want to caveat all of this: Having two antivirus engines installed when one of them doesn’t have realtime scanning disabled is an incredibly bad idea–they conflict with each other and stuff gets through. It may be possible to install other antivirus engines to coexist, but as of this writing, Avast is the only one I know of that is designed to do it.