I often get this question: "I want a free but good and reliable anti-virus software". I can honestly say that the AVG anti-virus package is not only free, but well supported and I have used it for probably over ten years and never had an issue with neither the software nor with viruses on my machine.
I however also have to add that I think we should support the commercial versions as those folks also have to make a living, and when comparing prices AVG is very competitive. Of course the commercial version offers a whole slew of features that are very useful that the free version does not offer.
You can find their official site here:
http://www.avg.com
The free version can be downloaded from here:
http://download.cnet.com/AVG-Anti-Virus-Free-Edition-2012/3000-2239_4-10320142.html?part=dl-avg_free_us&subj=dl&tag=button
See the list of protection the free version offers as compared to others:
http://free.avg.com/us-en/free-antivirus-download
Hope this helps. If you know of another great free anti-virus, let me know in this blog.