Sophos has offered a free Apple Mac OS X antivirus for a while. They now have a free Windows version. https://www.sophos.com/en-us/lp/sophos-home.aspx (Good for up to 10 PCs at home.)
** I have not used it at this point. I just wanted to make others aware it's out there. There are always questions about free antivirus packages...
Note the following caution posted at PCWorld: Researcher finds critical vulnerabilities in Sophos antivirus product
Thanks for the link!
A couple more thoughts...
The article makes a valid point in that Sophos was slow to respond to fixing the issues security researcher Tavis Ormandy (from Google) found on their antivirus product. Response repair time on critical bugs is important. But, considering the issue of software bugs, the fact that there were bugs/issues with Sophos's antivirus product is, unfortunately, far from alone. In the recent past, Ormandy has also found fairly serious issues with AVG, Trend Micro, Eset and Kaspersky (these are the ones I remember at the moment.) All these other issues have been, or are being, addressed as this is written.
Which just serves to prove that, IMO, reliance on an antivirus program for all of your/our computer protection is a mistake. Due to bugs or the way their components are designed/deployed, they will never be able to provide complete security.