I think u had Norton Antiviru previously,and some time ago uninstalled it,right.If it is,then Norton is not uninstalled properly and u have use the Remover supplied by the Symantec called RNav2003.exe for NortonAV2003 and similarly for other version,so download that utility and proceed as given in the teps there.
Visit for removal info:-
*service1.symantec.com/SUPPORT/nav.nsf/docid/2001092114452606