if you don't want to uninstall NAV the first thing you can try is to disable NortonCrashguardMonitor. you can do this by going Start>Run>Msconfig>click the Startup Tab> and unchecking NortonCrashguardMonitor.
let us know if it gets solved :)
if you need a good trojan scanner Ewido is good. for the first 14 days you get the full version - real time protection and automatic updates, they stop after 2 weeks and it turns into an on demand scanner. if during the first 14 days your PC is slow, right-click the orange E in the System Tray and disable real-time protection.
http://www.ewido.net/en/