function centerWindow( p_url, p_title, p_width, p_height, p_extra ) { 
	var xMax;
    var yMax;
    if (document.all){
      xMax = screen.width;
      yMax = screen.height;
    } else{
      if (document.layers){
        xMax = window.outerWidth;
        yMax = window.outerHeight;
      } else{
        xMax = 800;
        yMax = 600;
      }
    }
    var xOffset = (xMax - p_width)/2, yOffset = (yMax - p_height)/2;
    p_extra = (p_extra == null) ? '' : (',' + p_extra);
    return window.open( p_url, p_title, 'width='+p_width+',height='+p_height+',screenX='+xOffset+',screenY='+yOffset+',top='+yOffset+',left='+xOffset+p_extra);
  }