Go to the Scheduler folder and delete (or edit) the schedulings.
Or, if it is really a boot time scanning (not the one shown in the screenshot), this is how to disable scan at boot:
If you press F8 when starting Windows and select 'Last known good configuration' from the boot menu, the boot scan should not be started.
Or just run: C:\Program Files\ALWIL Software\Avast4\sched.exe /D