Have tried to install two free versions of MAC antivirus software. The installation completes successfully, but nothing happens. Using the Finder, I found the app but when I double click on it, nothing happens.
I tried AVG and then Avast. I finally deleted both, emptied the trash and rebooted my desktop. I downloaded Avast again and re-installed and it worked. I did get a message something about a "file extension". I said OK (have no idea what that was about) and then went in to Preferences and "allowed" the Avast software to do who knows what. In any case, it's running now. Thanks for taking the time to respond to my inquiry!