Hi Carmel,
Add/Remove does not uninstall Norton from your system. There are many things left in the registry that interfere with Avast!.
Please go to the Symantec WebSite and download their Uninstall program. After you’ve used that program to completely get rid of all traces. Reboot your computer.
Now go to ControlPanel … Add or Remove Programs… Select Avast! AntiVirus … remove and then Choose REPAIR.
Reboot again.
That hopefully should take care of your problems. Please let me know how you make out.