I installed Norton Systemworks 2004, which includes NAV 2004, last Wednesday and it was not an easy job. The program itself hasn't given me any problems worth mentioning, but the activation was a pain. Activation is necessary for the program to work after an initial 15-days period, but somehow my code wasn't accepted. Phone activation was necessary to make it work, but the screen you need for that only shows up once, during the first time you run the program. So I had to de-install the entire program, reinstall it and activate over the phone.
But, in the end it's working fine. I don't know how things would have gone if I'd just installed NAV instead of NSW, so I don't know if my story is of much use to you, but I just wanted to let you know.
By the way, I was using NAV 2002 before and I don't see much difference between that version and the 2004 one.
My dad uses a free anti-virus program that can be found
here. I have no experience with it, but he likes it.