Nortons AV is notorious for leaving traces of itself on the system after an uninstall through add remove programs. It even leaves services running, which isn't normally apparent until you try to install another security application which detects it.
Symantec (Norton) have a removal tool to completely rid the PC of any traces of Norton after an uninstall has been performed.
From the Symantec site (If you are sure which version of Norton is installed):http://service1.symantec.com/Support/tsgeninfo.nsf/docid/2005033108162039
From Major Geeks (Symantec is the author), Removes all Norton 2009/2008/2007/2006/2005/2004/2003 products, Norton 360 and Norton SystemWorks 12.0 from your computer:http://majorgeeks.com/Norton_Removal_Tool_d4749.html