< sorry for the pun >
I've also used Norton a/v reliably for many years, reinforced when others have been hit when I seem to have stayed unscathed.
When I went from 2002 to NIS (Internet Security) 2003, I was prompted strongly to uninstall A/V 2002 first. So when I went to install NIS 2005 recently onto WinXP-SP2, I'd uninstalled NIS 2003 first.
After disconnecting from the internet, of course. Then I turned off XP's firewall before I ran the installation ~ clean all the way. Reconnected up to the internet and registered, etc.
However, I can not say the same about Norton System Works or earlier versions of the components :-( Judging by the assertive manner in which I was adviced to uninstall previous versions, I would guess that you would to uninstall and then reinstall System Works after you have installed Norton 2005 successfully.
Hope that helps