The solution to your problem:

Use this 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

And use this uninstaller:

http://rejzor.wordpress.com/avast-cleanup-tool/

If you are using a paid version make sure…

Once you have uninstalled Avast… check if Device Manager if there are any NDIS firewall avast drivers. If they are remove them and then reinstall.