SOLVED!!!
Well, the problem but not the mystery...
After many(!) hours of troubleshooting I finally found what was causing the NAV to shut down.
in the: "documents and settings\[my username]\local settings\temp"-folder (Im running a non-english version but I think those are the names of the folders) I found two files:
"00000622" and "00000646" each 894 kB. When I scanned them NAV shut down. They were marked as archive-files in the properties. No extensions and no info about them otherwise. I tried to open one in Word (maybe not the smartest thing to do...) - then Word shut down just like Norton did when it tried to scan them. I can scan the rest of the computer now, just fine as long as I dont touch these two.
I also found a suspicious looking file called wc_bundle.exe, the file passes the NAV-scan, but since I did have the backdoor-virus Im thinking that this might be related. The properties description is "Raven DR-Fetcher Executable", and the company is eAcceleration Corp.
On the first look, this appears as a setup program for Webcelerator - but can this be a modified version that was dropped by the virus? Raven DR-Fetcher Executable...hmmm?
Is there any way of inspecting these 2 strange files that caused the crash?
Im so happy to have solved this mess...I hope.