As far as I'm aware there is no known issue with the avast splash screen, there is however an issue with Norton spit and getting rid of it, this can really mess with an installation of other security/AV programs.
Because of this NAV removal issue avast may not have installed correctly and this could result in many different issues. If a repair as rassel suggests doesn't resolve it try, uninstall avast, reboot, install, reboot.