Well several companies, such as PC magazine and PC World, have investigated norton and it's integration into a PC.. it burries itself into C:\Program Files\Symantic, c:\Program Files\Common, C:\Windows\System32, C:\Documents and Settings\All Users, C:\Documents and Settings\(Username), and other locations on the HDD, thus slowing system performance when it is being used as active scan while the system is running, dropping system performance by up to 30%.. It has also been recorded as itentifying some legitimate program files as Virus and deleting them, as such leaving the program inoperatable.. this includes some of the more obscure Microsoft published programs.. Plus the deep rooted Registry entries.. on some systems, it isn't so bat, but any program rooting a minimum of 10 registry folders (not the keys them selves.... Keys number much more) tends to be considered invasive..
On top of all that, Norton does not fully uninstall using Add\Remove programs..
if one wishes to step up to a newer or better AV scanner, you will have to manually delete the Symantic registry entries.. the norton registry entries, and all folders located on one's system prior to installing any other AV.. the method is so troublesome that it is designed to discourage anyone from uninstalling the invasive norton program so that people will re-install it in order to keep some protection on their system no matter what impact it has on their system...
Most other Anti-Virus company programs will uninstall almost completely using Add\Remove programs and will automatically uninstall the registry keys and *.dll files where norton will not..
Norton is not an actual virus, but it is reguarded as a Virus by several companies due to it's invasiveness on a system and the impact on system performance.
It took me over 2 hours to completely uninstall Norton System Securities 2k3 off my sisters computer just so that Panda Internet Securities could install...
When she decided to try AVG, Panda uninstalled in 7 Min without a problem one and AVG installed just fine...
then she decided to go back to Panda as their software had more to offer and protected her system against much more than AVG, the change back took less than a total of 15 min...
It depends on what people like to use, but i tend to go with performance sites because i am a system speed freak (without overclocking my system.. i don't like burning up my system)... After doing my own testing with norton and other AV's with my sisters system, I really noticed a performance drop when it was running norton... and the pain it was to uninstall.. it was not good... norton imbeds so deep in the registry, that even if you have a good registry cleaner like RegSupreme Pro, RegSupreme can not remove the Symantic or norton entries from the registry after norton is uninstalled.. the entries have to be manually removed..
to me that is just a bit too invasive and the performance hit of 11% on my sisters system due to norton, it really put me out..
Anyhow.. hope that this helps.