When you download the setup.exe for Avast (or, any downloaded file, for that matter), you should unblock the file before installing Avast (before running the setup.exe). Running "as Administrator" usually helps too.
Reading the complete topic, I see I gave you all the "correct" tips to avoid this type of problems before installing.
My *suspicion* is that you are not installing Avast in the "correct" way, but of course the possibility that some malware can be also interefering is always opened.
Someone else should continue from this point on.