Norton and Symantec products leave remnants even after uninstalling.
These remnants can and will cause varying degrees of conflict. Seen it on this (and other) forums before. A lot.
If you didn't use the Norton Removal tool, chances are high it's the reason.
Avast also has an [urlhttp://www.avast.com/eng/avast-uninstall-utility.html]uninstall utility,[/url] which would be advisable to run before re-installing the program, as it is possible your installation is now corrupt.
It is almost worse than the virus trouble. I do not understand why a company would write a program that you can not uninstall short of formating the hard drive
Some ex- Norton users have been known to voice similar sentiment about Norton.