From what I've seen, these security programs like Norton and Mcaffee, pretty much take control of your entire computer and have influence over every program a user runs. This can be a MAJOR pain in the neck when it comes time to use some of your common everyday programs. Just the other day my friend couldn't use his AIM instant messenger because Norton was blocking access. Another time I was fixing a relative's computer and saw the pure devastation of Mcaffee virus scan. Its combined running processes took up an excessive amount of resources and many programs did not function because of the security settings. Not only that but software like Norton is easily damaged or disrupted if you close or disable any of its processes. Also it can be incredibly difficult to uninstall (as I've seen many times) and often traces are left behind which can cause problems.
To the average every day user many of these security programs can become a nightmare if they do not know the proper way of re enabling a program that was labeled "dangerous". They immediately assume that a virus has caused the problem when in fact it is the program they are using to guard against them! Sad how so many people are fanatical about using them.
Firefox + common sense + weekly scan