If you have a computer that cannot at all handle heavy-duty software, I advise Avira free antivirus.
Otherwise, AVG Free is the best in the market. I've tried both of these, with good results.
Right now my laptop came preinstalled with McAfree, but as soon as the license expires, I'm downloading AVG once again - its one of the best, and trust me when I say this, I've been using it for the past 8 years without any problems.