I believe that by default avast doesn't start in safe mode, this is to allow a way into the system if there happens to be any conflict (e.g. you have another AV and you installed avast also and the system locked for instance).
There should be an avast! antivirus shortcut icon on the desktop which you can run avast from safe mode, or run this file, "C:\Program Files\Alwil Software\Avast4\ashAvast.exe"