It should disable itself automatically (it should run only once after you schedule it), doesn’t it behave this way ?
Could you please send me the file C:\Program Files\ALWIL Software\Avast4\Data\log\aswBoot.log ? (to peterka@asw.cz)

See also this thread: http://www.avast.com/forum/index.php?board=2;action=display;threadid=874