By the way, today I tryed to do this, and surprisingly, it works!
I just had to deselect the antivirus to be installed with SystemWorks (to prevent duplication with one fron Internet Security).
Everything is fine, except one problem, while I open Mirosoft Outlook 2003 (Norton AntiSpam enabled), Windows Installer trying to install something, and then an error pops: "Norton AntiVirus 2005 does not support the Repair feature, please uninstall and reinstall."
Does anyone know what does this error actually mean, and how to deal with it (I really don't want to reinstall it again). So maby any other suggestions?