I can understand not having money but getting a proper AV like Bitdefender is paid back by peace of mind. Between it and Ublock Origin add on for browsers, I can't even visit sites that would try to give me all these lovely add ons and unexpected extras. Its the best cure for all these things you pick up.
Its a shame Bitdefender free doesn't work with win 10 from what I can see. Avast free is okay except like their paid versions, it always feels like they want your money.