The main users guide is the avast help file.
Windows Start, All Programs, avast! Antivirus, Help or C:\Program Files\Alwil Software\Avast4\ENGLISH\HELP\help.chm. Or from the menu in the Simple User Interface (on-demand scanner).
You can also browse the forums, especially the sticky topics at the top of each of the forums. They provide a wealth of information to help you get the best from avast.