Try a clean install again but this time use the online installer :
Avast Free: http://files.avast.com/iavs9x/avast_free_antivirus_setup_online.exe
Avast Pro: http://files.avast.com/iavs9x/avast_pro_antivirus_setup_online.exe
Avast IS: http://files.avast.com/iavs9x/avast_internet_security_setup_online.exe
Avast Premier: http://files.avast.com/iavs9x/avast_premier_antivirus_setup_online.exe

I don’t know why but I had problems with the offline installer but not with the online one… it doesn’t serve any technical difference.

Avastclear : http://files.avast.com/iavs9x/avastclear.exe