Also, I'm assuming you still have Norton installed on the same system where you run avast! just because you haven't mentioned you uninstalled it. You should be aware that two resident antiviruses installed and running on the same system in the same time, will give you nothing but problems. They simply can not work together, and it's not the case with avast! only, but with any other resident antivirus program.
Uninstall (if you haven't done that already) Norton completely, and I mean completely. There are hundreds of threads in this forum (use search feature, button up there) dealing with Norton uninstallation procedure, plus they provide links for all those removal tools for your particular version of Norton. Once it's completely wiped out from your system, you can reinstall (from the scratch) avast!