If you go o here http://thewebatom.net/uninstallers/security-software/ and run the tool to delet any leftovers of your previous AV (22a in the list), there are tools there for any other AV’s you may of had in the past as well,
Reboot after running the tool and then try a repair of avast via remove programs.
You can also set avast to start after other system services by going to settings\troubleshooting.