Avast should start automatically on boot, normally there is an avast Icon on the desktop (and some don't like that, clutter). It is for the AvastUI.exe file not the main program but to allow you to access the UI.
There should also be an avast icon in the Notification Area, System Tray (bottom right of your screen), if it isn't there it could be hidden, you just need to drag it into the tray.
Opening the AvastUI allows you to add or remove components.