Can you see if you have an entry for uninstall in the Control Panel?
Can you see if you have the avast folder into the Program Files\Alwil Software directory?
It's not a bad downloaded file as you are not receiving the error 'broken package'...
I suggest, if nothing else work, download the application under 'Control' in my signature and choose the option to 'Perform forced uninstall' or use avast! uninstall utility (
http://www.avast.com/i_idt_1336.html). If you can't uninstall avast through Control Panel, you can download the uninstall utility called AvClear or AvClear4 (depending on your avast! version).
Then you can try installing it again