There were a bug on boot time scanner (http://forum.avast.com/index.php?topic=29999.msg247134#msg247134).
You’ll need to update for the last virus database (VPS), which is not present in the website setup file.
After doing that (updating), try to schedule a boot time scanning again if you want.
Right click the ‘a’ blue icon, choose Update > Virus Database (VPS)
Start avast antivirus.
Click on the Menu button.
Choose Schedule Boot Time Scan.
Doing so displays a dialog allowing you to schedule virus scanning.
Check Archives, if you want scan all the archives.
Specify whether all the disks or just a specific folder should be scanned.
Select Advanced options for scheduling details.
Select how to automatically process infected files.
Choose how to automatically process infected system files.
Click the Schedule button to confirm the settings.