Tente instalar novamente e quando der problema, abra o arquivo C:\Program Files\Alwil Software\Avast5\Setup\setup.log, copie as últimas 350 linhas dele e cole-as aqui.
Try to install again and when you've got a problem, open the file C:\Program Files\Alwil Software\Avast5\Setup\setup.log, copy the last 350 lines of it and post them here.
Sorry, this is an English-only forum.