Try opening avast then look at the drop down menu there is a heading "Schedule Boot-Time scan" this opens a settings window, though my understanding was that it only set the boot scan to run once. Try clicking on cancel, see if that works.
Can't help you on this one I'm afraid. Tried Avast for 36 hours and came to the decision that anyone who uses it cannot have too much use for their computer, except for a little time after work or whatever they do.
This may help. If you open the Avast main screen the simple user interface then click on the menu arrow in the top left hand corner. Click on Schedule boot time scan. It should then say it is set and do you wish to cancel the scan, click yes, it should then stop further boot scans. W