You will need to access the stage.
Import the needed classes.
Actionscript Code:
import flash.display.StageDisplayState;
import flash.display.StageAlign;
import flash.display.StageScaleMode;
import flash.events.Event;
Set your registration point for the stage. And set the stage scaleMode.
Actionscript Code:
stage.align = StageAlign.TOP_LEFT;
stage.scaleMode = StageScaleMode.NO_SCALE;
Now when a user resizes the window objects will not move nor resize.
To make the background resize position it on the stage, and make sure it has it's registration mark in the top left. Place this Background MovieClip on the stage at x:0, y:0.
Add a listener for the stage resize event.
Actionscript Code:
this.stage.addEventListener ( Event.RESIZE, this.resizeBG);
Then add your function that resizes the MovieClip, I call it "BG" here
Actionscript Code:
private function resizeBG( evt : Event ) : void {
var w : int = this.stage.stageWidth;
var h : int = this.stage.stageHeight;
this.BG.width = w;
this.BG.scaleY= this.BG.scaleX;
}