I can't find the exact explanation in the millions of emails that I have -- but it sounds vaguely like a problem I had months back with the IRP Stack size being set to low - (Norton doesn't play nice with alot of applications)
Try boosing the IRP Stack and see if that helps.
the number should be between 4 and 12 - -i think 7 is often the default
http://www.windowsnetworking.com/kba...stacksize.html
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Next thing - have you tried uninstalling NAV completely and does it work then?