Free antivirus programs are exactly the same as the antivirus component of the full 'security suites' that the manufacturers hope that you'll go onto buy. So, as long as you've got the 'extras', such as Malwarebytes, anyway, there's no point paying for an antivirus program.
So the obvious question then is "Which one?". I'll try to answer that for you:
Avoid Microsoft Security Essentials. It failed several independent tests last year and Microsoft subsequently admitted that it's not as good as they'd like it to be.
AVG has plenty of fans (and I've used it myself in the past) but I've found that it slows my elderly computer to a crawl.
Avast! is possibly the most popular freebie among ABers, and it's what I use:
http://www.avast.com/index
Comodo Antivirus is a relative newcomer to the freebie scene. (The company has given away its firewall for many years but it's only fairly recently that they've started giving away their antivirus program as well). I've no experience of it but it seems to be well-reviewed:
http://www.comodo.com/products/free-products.php