After using the avast! removal tool, did you reboot the system before attempting to install avast! again ???
If that wasn’t done, please follow the instructions outlined here:
https://docs.google.com/document/d/1TCCX0R7AAF2WOxAMQ_kcun2nNnCPFAk2P4RBzFXSgds/edit?usp=sharing