Hi,

in the Home version you can do it via editing the Deftasks.xml; see the User’S FAQ:
http://forum.avast.com/index.php?board=9;action=display;threadid=4818;start=15

in the Pro-Version it should work via editing the tasks in the GUI

you might also browse the topic about avast4.ini:
http://forum.avast.com/index.php?board=2;action=display;threadid=1647