jQuery.fn.centerScreen = function(loaded) 
{
  var obj = this;
  if(!loaded) 
  {
    obj.css('top', ($(window).height()/2-this.height()/2) + $(document).scrollTop());
    obj.css('left', ($(window).width()/2-this.width()/2));
    obj.css("margin-left", 0); obj.css("margin-top", 0);
  }
  return this;
};