I am using free AVG - great ratings from independent AV testing labs. I would stress using on demand scanners like mentioned - malwarebytes. I am using it too. It is very good thing to fight PUPs like the one I got recently - called multiplug I do not remember installing it, probably it came bundled in installation file together with other soft. Suddenly ads started appearing across visited websites, I scanned with malwarebytes - found and removed multiplug.