Do you have any other antivirus installed in your system?
Did you have in the past?
Which one?
Can you try to repair your installation?
Go to Control Panel > Add/Remove programs > avast! antivirus > Remove
Then choose Repair function in the popup window (Repair).
If this does not help, can you uninstall / boot / install / boot again?
You can add that registry key manually but I think the steps I posted before should solve your problem.
Last one, can you check if you have the value ShowTaskbarIcon=1 on the [AAVM] section of the avast4.ini file? See ‘Settings’ on my signature for more info…