ZDNet: Story: Antivirus programs: One is not enough
IF YOU HAVE ONLY ONE antivirus engine, you depend on that vendor to continually update its virus definitions and to be able to immediately identify virus-like or worm-like code when it arrives in an e-mail. Unfortunately, no antivirus engine has a perfect record. You're running a small but measurable risk.

"Every engine can have an off day," says GFI CEO Nick Galea. By using multiple engines, he says, you make the virus writer's job much more difficult. The same is true for hackers who try to slip a worm onto your network to gain access. "The chance is much smaller" that they could get past multiple engines, Galea says.

And that is the reason why GFI supports the use of up to three antivirus engines at the same time. The BitDefender and Norman antivirus engines, which are very popular in Europe, ship with MailSecurity. A third, from McAfee, is optional.

Galea thinks that running three engines can give an enterprise a virus catch rate of better than 99.9 percent. "You can go years between successful virus attacks this way," he says.

Of course, having great antivirus protection on your e-mail gateway doesn't mean you can abandon your other antivirus software. You still need to have an antivirus package on every workstation because some users will use disks of dubious origin, visit virus-laden Web sites, or do one of many other things that may put your network in jeopardy. But the single biggest pathway for viruses is still your e-mail. And you can protect that.
http://www.zdnet.com/anchordesk/stories/story/0,10738,2877016,00.html