Well, Counter seems to be the only person here today that seems to have somewhat of an intellectual answer.
But, to answer your question, the TOP FREE AntiVirus is from Avira, entitled Avira AntiVir.
The antivirus comes with:
>Real-Time Protection Features
>State of the Art Anti Rootkit Scnanning
Avira AntiVir's Malware Detection rate is varies from 97 to 98%, whereas Avast!'s Malware Detection rate varies from 80% to 85% and AVG's varies from 60 to 80%. In addition to AVG's horrendous detection rates, AVG detects around 25% false positives. This means, that 25% of the 60-80% is false positives, leaving you with a 35 to 55% solid detection rate.
My research is backed-up through http://www.av-comparatives.org/.
Here's an example of the August 2007 On-Demand Scan Test: http://www.av-comparatives.org/seiten/ergebnisse_2007_08.php. If you look at the column that shows the Polymorphic Viruses (These are like, the worst of the worst viruses): Avira collected and destroyed 11 out of the 12, whereas AVG only found and destroyed 3 out of the 12.
All in all, Avira in my honest opinion, is the BEST Free AntiVirus software.
You may download what you wish, but: "What's popular is not always right (AVG is the most popular Free Antivirus solution) but what's right is not always popular (Avira is a little, less known but definitely does a better job!)
I also want to throw in System performance, AVG impacts system performance significantly, but Avira barely does!
If you need any help, I would be more than happy to help. You may reach me via the "My Profile" section of Yahoo! Answers.
The link to Avira is: http://www.free-av.com
Hope I helped,