There are many good free alternatives to paid antivirus. There is AVG, Avast, Bit Defender, Avira, to name a few. Whatever you pick out of these, then use MalwareBytes as a manual scan. The free version of MalwareBytes can only be used as a manual scanner / removal tool, but it's very good at detecting and removing viruses that are difficult to find / remove. Used as a back up to your main AV solution, it's a deadly combo.
Of course the primary AV solution is the user, so as long as you are smart, you should be OK. However it occasionally happens that you'll mess up and a virus will get one past you, this is where your main AV should help to save the day.