I`ve just uninstalled Norton from my puter and installed Avast instead. Avast found 2 Trojans on my system which neither Norton or AVG found.
Fingers crosssed but I don`t appear to be getting the black popup now.
Apparently uninstalling and reinstalling Norton fixes the problem but all folders relating to Norton need to be removed as well.
Here`s the link for Avast Antivirus, free, if it`s needed.
http://www.avast.com/eng/avast_4_home.html