I am using NAV 2002, didn't face any such problems.
Updates not installing would happen
* If the Subscription period has expired. Check it with that. You could also try doing LiveUpdate.
* A Virus which may have infected the system
If not then have a look at the following site which is all about NAV issues:
http://www.computergripes.com/NortonAntiVirus2003.html
If above didn't work, try to do a Re-Install & check for the update thing.
Also, a last resort would be that you may have to install some trial versions of Kaspersky / AVG (free) to check for viruses.