Save the uninstall tool and run it on the Desktop. Once that is done, restart the computer. Then download the correct version of Avast and install it.

http://files.avast.com/iavs9x/avast_free_antivirus_setup.exe - Free
http://files.avast.com/iavs9x/avast_pro_antivirus_setup.exe - Pro
http://files.avast.com/iavs9x/avast_internet_security_setup.exe - Internet Security
http://files.avast.com/iavs9x/avast_premier_antivirus_setup.exe - Premier