Hi mbhkoay,
We are sorry for your bad user experience. While we understand this may feel frustrating, we would like to inform you that we never install software without your consent. What probably happened, there had been a small check box ticked by default, opting you in for the browser installation, that did not get noticed during a program update of Avast AV.
Of course there can be a flaw in the process but we are currently not aware of any. We can try to analyze log files if you are willing to provide them to see what exactly happened.
You can also uninstall the browser any time via Control Panel or Start menu.
Have a good day.
Pavel @ Avast