Your best bet is going to be to read the EULA that accompanied the software.  Since they've changed the product activation requirements/procedures between 2003 and 2005, there may be different restrictions, but by way of example, the EULA for NAV 2003 reads in part:

"You may:
A. use one copy of the Software on a single computer.  If a License Module accompanies, precedes, or follows this license, You may make that number of copies of the Software licensed to You by Symantec as provided in Your License Module. Your License Module shall constitute proof of Your right to make such copies.
B. make one copy of the Software for archival purposes, or copy the Software onto the hard disk of Your computer and retain the original for archival purposes;
C. use the Software on a network, provided that You have a licensed copy of the Software for each computer that can access the Software over that network; and
D. after written notice to Symantec, transfer the Software on a permanent basis to another person or entity, provided that You retain no copies of the Software and the transferee agrees to the terms of this license."

Whether or not this means you can legally sell the product as a stand-alone item isn't clear, and since you've already activated it on a different system, that may make things more difficult.

Since support for NAV 2003 is minimal at best now, I think that instead of selling it, I'd just uninstall all the fluff and install only the NAV portion of it.  That way, you still have a current AV app, and you don't have to put up with all the completely unnecessary (for Windows XP, anyway) other components.

You can locate the EULA for your product by navigating to C:\Program Files\Norton (or Symantec) System Works.  The EULA should appear as a text file, possible under the name of end_user.