If it is a conflicting program that's the cause, it will most likely be a similar product such as another antivirus application, or even an earlier version of a Norton Antivirus. Any of these must be fully uninstalled before trying to install NAV 2005.
In the case of an earlier version of NAV having been installed and not completely removed, it's notorious for leaving traces behind on your PC even after uninstalling it. There's a guide here on how to remove the bits that are left over:http://sniptools.com/tutorials/uninstalling-norton-antivirus
Even redundant registry entries are enough to make the installer think that a conflicting program is still present on your system. It might pay you to open the Registry Editor and run it's 'Find' tool to search out and delete all entries for 'Norton Antivirus', then try installing NAV 2005.
Edited by pip22, 30 December 2006 - 08:08 AM.