Picking up on Panda Anti Virus, all my business customers use it. They do a free cloud anti virus which I install for people who don't want to purchase any AV software. It is currently Number 3 in the download chart on filehippo. I used to install AVG as a free AV, but it kept finding to many 'false positives' and the filesize was huge. Panda is small to download, lightweight on system resources and pretty simple to use.
I would reocmmend using this, and once every few weeks running a scan with malwarebytes, again, you can get a free version of this.
You can get Panda, paid or free version from
www.cloudantivirus.com