There is some very exciting news from Sophos — Sophos Home has launched (in beta) for free, for non-commercial use. It is licensed to never expire, and it works on Mac and Windows. This is very good news for every home computer!
If you need a secondary antivirus software, check out Immunet:
Please note that you should check the terms of service for these free versions to make sure your use case is appropriate for the free license.
* Every business or corporation or commercial use should have a professional security package installed and administered by a reputable network administration and security firm (like Your Computer Genius). Free antivirus programs may be suitable for the home user.