¿Is there any option in this software to remove this scan forever?
Generally when you schedule a boot time scanning for the second time, it will ask you if you want to disable the first one.
It's not a permanent option so you must have a trouble... It shouldn't be at all boot time, but just one.
Can you schedule a boot-time scanning twice and see if you receive this message to disable it?
Start avast! > Right click the skin > Schedule a boot-time scanning
In XP, if you press F8 when starting Windows and select 'Last known good configuration' from the boot menu, the boot scan should not be started.