Is this a new or expired edition of norton?
once it's more than a year old they can get hungry for more dosh
Have you just installed it?
There are usually two ways to download the updates. Live update, and download manually from the site
The fault is more likely to be the norton installation rather than the isp
If all else fails, remove and try again. If you haven't paid too much for it, uninstall and download free AVG. It's much less fussy and much easier to use