This is possible only in Professional version (see picture here:
http://forum.avast.com/index.php?topic=13315.msg112285#msg112285).
In Home version you can check the option "Don't show this window again" as soon as the first virus warning appears, and click on "No action" button. This way, nothing will be done and you will be presented the results at the end (and you can perform actions from there).
Well, that's the way it is, I am afraid; you may call it a limitation of the Home Edition of avast!. In the Professional Edition, you can configure the task to perform automatic actions (or, don't do anything).
In the Home version, you may check the "Don't show again" checkbox in the Virus warning dialog - but only after the first virus has been found (because you won't see the virus dialog until then).
