I "think" you can ONLY schedule a boot-time scanning.
Start avast! > Right click the skin > Schedule a boot-time scanning.
Select for scanning archives.
Have a look here: http://forum.avast.com/index.php?topic=35347.0
hello this my sound strange but i am using avast 4.8 what i am wondering is can i program avast 4.8 to search for virus's during the night time if so how do i do it?