I've found it. Not sure if its working for Avast! Home but it works for Avast! Pro and you can try it for 60 days...
-install Avast! Pro the way you like
-configure it the way you like
-Launch it by clicking right mouse bouton on the systray icon, and choosing "Start Avast!" (not by double clicking the icon, this would open another window)
-Click the uper-left icon of the window and select "Go to advanced gui"
-Now, on the advanced gui, click on File, and the "Create silent installation"
This will create 2 or 3 files: a .xml and .ini files, and a .dat if you protect avast with a password (this would be your password encoded in the files)
-Copy those 2 or 3 files in the same directory as the setup file.
-Run the setup file, without any parameters. This should install it silently
thats it!

Look in the help file for more info.