Actually there isn't. This is a limitation of the Home (free) version that it has interactive input requirement, there have to be differences in the Home/Pro version and this is one of them, the programmers have to eat
Check this work around out - HOWTO: Scheduled Automatic Scans avast! Home Version!
The major disadvantage with the task scheduler trick of using ashquick.exe is, it will scan every file of the hdd, partition, folder or file that you set it to scan; even those files not considered a potential threat, this can take a long time. It is possible to add certain folders to the exclusions list for say your media files, etc. Care should be taken with the paths you want to exclude as this could leave you vulnerable if that folder got infected as it wouldn't be scanned by the on-demand scanner as you have excluded it. However, should you access any infected file in a Folder in the Program Settings, Exclusions, the on-access scanner (Standard Shield) should detect it provided you haven't put the exclusions in there.
The Pro version gives a lot more flexibility and the scan would follow the settings and exclusions, etc. of the regular scan.
With the different levels of protection (Web Shield, Internet Mail, P2P and Instant Messaging) and the fall back of the Standard Shield, I find the need for daily scheduled scans lessened. I prefer to do a manual scan with my regular weekly maintenance tasks.