Which version of Norton do you have?
From, I guess, 1st January Symantec have stopped supporting SystemWorks with any version prior to 2004. If you want to use LiveUpdate then you must upgrade to the lastest version.
Alternatively, you can manually download and install the virus definitions from here:
http://securityresponse.symantec.com.../download.html
Edit:
RTFP
"NAV 2004 Pro" - dunno then. Still you could try the manual download.