Have you tried to un-install and re-install NAV?
Have found that this works after repair/install of O/S.
Have 2004 Professional, has worked well until yesterday.
To fix another problem, I repair-installed XPHome and downloaded the 14 MS critical updates. After re-boot, NAV popup said to get updates, but the LU1803 screen (LiveUpdate failed) popped up after every one of several attempts.
Ran the Norton site's virus checker (which does not scan compressed files) and it said all was OK as far as it went. Fine.
Back to NAV site, tried to download the August 12 update for Automatic Update (the 'lite' version.) The dialup ground away for about 15 minutes, then the MS "page unavailable" screen came up. Retried same download several times, same results.
Getting through to Norton is not an easy option.