The install failed, but it may have managed to do some of the installation before it failed, so you need to get rid of that failed install. The default location for avast is C:\Program Files\Alwil Software\Avast4, check to see if that was created.
If you can't do it with a regular add remove programs (with is likely because of the failed install) then you use the uninstall utility and continue from step 3 of Tech's Instructions which includes a link to download the avast Uninstall Utility.
We are trying to help you so the CAPS LOCK key is the loud (shouting) key, yes you might well be frustrated, but we do this in the hope that it might help no other reason.