Timilian, can you please try to download and install avast using online installer. Please, report back.
Download locations:
http://files.avast.com/iavs9x/avast_free_antivirus_setup_online.exe http://files.avast.com/iavs9x/avast_pro_antivirus_setup_online.exe http://files.avast.com/iavs9x/avast_internet_security_setup_online.exe http://files.avast.com/iavs9x/avast_premier_antivirus_setup_online.exe