CraigB
2
Try a clean install of the latest version of avast rather than upgrading and see if that makes a difference.
Get the version you require here http://forum.avast.com/index.php?topic=89262.0 and save then go here http://www.avast.com/uninstall-utility read and follow the instructions to remove avast in safe mode followed by a reboot and then install the new saved version of avast, register then reboot again.