When I started my job a little over a year ago, we had Norton Corporate Edition, fully up to date installed on our internal server. Browsing through some files, I came across about 4GB of German um... movies, in a hidden folder (read as: server got hacked). I managed after about two hours to get Norton uninstalled and put the free version of AVG on the server to test... found over twenty trojans and viruses. We have since moved to a mix of AVG Corporate on our XP systems and Avast on our Vista systems. This gives us some overlap so if one antivirus misses something, the other should catch it.

Also, because viruses are modified quickly to avoid detection, most antivirus products use heuristic scanning to find new viruses, and people who write the viruses test against the most popular programs, in this case, Norton and McAffee. For the same reason Windows has more viruses than Macs (most people use Win, so that is what you attack), more viruses get through the big two AV products than the smaller ones (again, you target what most people have).

Every once in a while, obscurity helps in your security.

__________________
The best way to learn anything, is to question everything.