Registry cleaners are not necessarily a good idea and can sometimes do more harm than good, but a full reinstall may be your answer -

Reinstalling

a) Download Avastclear, Rejzors uninstall tool and an Avast program edition

http://files.avast.com/iavs9x/avast_free_antivirus_setup_online.exe
http://files.avast.com/iavs9x/avast_pro_antivirus_setup_online.exe
http://files.avast.com/iavs9x/avast_internet_security_setup_online.exe
http://files.avast.com/iavs9x/avast_premier_antivirus_setup_online.exe

Avastclear : http://files.avast.com/iavs9x/avastclear.exe
Rejzors Uninstall tool: http://rejzor.wordpress.com/avast-cleanup-tool/ (Cleans up traces possibly left by Avastclear).

b) Uninstall Avast via control panel
c) Uninstall using Avastclear. (it will ask to be booted in safe mode. Say yes) and wait for normal mode to reboot.
d) Run Rejzors Uninstall Utility and reboot.
e) Install the version you downloaded.
f) Reboot, may need a couple of reboots to get everything settled in and working correctly.