I used to use AVG but I'm not sure if they offer a free service anymore, but I've also used Avast and that's pretty good, and also free. Here's the link to that one so you can check it out: http://www.avast.com/eng/download-avast-home.html

I personally don't like Norton, but that's just me. I have McAfee running at the moment, and I'm happy with that.