When attempting to update using Norman Internet Update an error like the following appears:
The download directory C:\Program Files\Norman\Distrib\Download is in use by another NIU
The process NIU.EXE (Norman Internet Update) is already running, often from another account than the one you are using now (mostly the Local System account, from which the automatic updates are executed). That other NIU.EXE instance is awaiting user input on some unanswered popup, and as long as that NIU.EXE is running it keeps a file called NIU.SEM locked in the indicated directory. If that NIU.SEM is locked, other NIU.EXE instances will produce this error message.
If you do not see the popup that the already running NIU.EXE is presenting (for instance, it shows popups when your license is expired or when you are trying to download updates that are not covered by your license), it means that the popups are presented by NIU.EXE on the console, and you are logging in remotely and thus do not see the console messages.
Note: Completing the updates after downloading can take a couple of minutes.