Bite the bullet and do a full clean install of the latest beta i think http://files.avast.com/files/beta/6.0.1279/setup_av_free.exe i believe you already have the uninstall tool?
Delet avast via remove programs and reboot into safe mode and run the uninstall tool the reboot back into normal mode and run ccleaner then you can install the new beta, see how that goes.
Its also a good idea to do a manual search for any leftovers of norton as even after running there uninstall tool there will still be some stuff there, it pays to run the norton removal tool twice as it’s a bugger to get rid of, type Norton in the run bar then see more results then click on computer to do a search, repeat the process again but type in Symantic.
If your comfortable in the registry you can also run searches there under the two name’s i listed, i know for a fact there there will be still quite alot in the registry