Seems like you tried everything reasonable so here's a brute force approach, one that comes with a big disclaimer - it might get you into more troubles than you have right now:
- Boot
- Kill all processes related to Avast in the task manager
- Remove all references to Avast from your registry
- Delete the Avast directory manually (as much as you can)
- Reboot and sort out the errors coming up during reboot, probably calls for DLLs you deleted in step 3.
Again - this can lead to trouble, up to your machine no longer booting at all and requiring extensive troubleshooting. But I have been in a similar situation with Avast and it worked out.
Maybe the more knowledgable folks have something less drastic to suggest?