Free antivirus vs. paid? Let FREEdom ring!
This article suggests that the major free AV software (AVG, Antivir, Avast and even the lowly much maligned free Microsoft Windows Defender) correctly detected the latest (12/11/2008) zero day virus. Mcaffe, Norton and ESET Nod32 missed it!
http://www.informationweek.com/news/internet/security/showArticle.jhtml?articleID=212400523&subSection=Vulnerabilities+and+threats