I suppose that you reinstall avast! because this is not a bug but, probably, a problem of your previous installation...
Usually, if you try to schedule a boottime scan and want to cancel it, you can try to do the same thing again... The 'second' tentative of scheduling will ask you if you want to disable the boottime scanning.
If you cannot reach the final of the boot, so you have a problem in the windows registry. The key I cannot write now because I'm on a ME system right now... But can be manually corrected (Igor could guide you or you can find a forum, seaching for boottime problems).
A well-done installation of avast! works and do not avoid the end of the boot. The boottime scanning could be easily canceled or finished as well.
Oh, at least, this is the normal behavior in my computers