One of my favorite blogs, Bruce Schneier, points out that virus writers are constructing their newer viruses to avoid the most popular antivirus programs. If that becomes a significant trend, moving to a lesser known product may actually provide more security.
One of my favorite free systems is ClamAV . It is a great system for a Linux server. It runs on Windows but it does not have real-time scanning. It does have real-time scanning in some versions of Linux. Why real-time scanning is needed for a Linux desktop is not clear to me. There are very few viruses targeted at Linux computers. ClamAV is a great solution for an email server.