Hi! Mine is an corporate edition ( 2007 i think) and since i've installed Windows Service Pack 3 i get the same problem. This is my home computer , so i don't have limited acces or anything like that , but today i think i've just foud the solution for this problem: first i've scheduled a custom virus scan from "Scheduled Scans/ New Schedule Scan , then I just close everything that had to do with NAV from Task Manager .
Then press " Start / Run / {"C:\Program Files\NavNT\vptray.exe"}-in my case-.
After this, the Virus Scan started immediately and found some viruses too ( Suspiciuos.Vundo.2 , Trojan.Vundo ... ). Hope this helps you !