May be completely off beam, but check to make sure that no malware has got in, as some of them now can deactivate things like Norton so that they can do their worst. Spybot may be worth a download to make sure that there's nothing nasty going on.
Have you tried making NAV run live update manually?
I've seen situations where NAV gets well upset if the previous Liveupdate fails to run clean, and it can get messy sorting it out.
There's an option in Liveupdate for "more detailed error reporting", make sure that's on, and also check the options and set it to not use express mode for now, until it's sorted and clean.
That's a starter, best of luck with it.
Cheers
Steve