AVG or AVAST seem most popular
i like the free ones like kaspersly and mcafee that you can get from banks. although they do expire
I have that part of my anti virus and internet security, Malware Bytes and Panda at the moment, panda seems to perform quite well in a few reviews whether they are expert reviews and even the odd user opinionAvg is awful.
Malware bytes is a must, and free. Run a scan every so often