I think I had a similar problem by last week. I was not able to escape avast scheduled boot-time scanning page and it was not scanning in the first place, there was a malfunction.
I rebooted my pc manually, chose to start with "Last known Good Configuration" instead of "Start Windows Normally", booted to windows and reinstalled avast. Not sure though if there is another way to "disable boot-time scan" but that was my only choice.
Make sure you download avast setup and the lastest update manually from avast.com before deciding to uninstall and that you are disconnected from the internet while uninstalling.
