Before installing avast!, you first need to totally remove any other AV program on your system.
You can only have one resident AV on your computer. Two is not better and as you found out only leads to a conflict.
You’ll find step by step instructions on how to properly install avast! at:
https://docs.google.com/document/d/1TCCX0R7AAF2WOxAMQ_kcun2nNnCPFAk2P4RBzFXSgds/edit?usp=sharing
Let us know if you have any other problems.