function centerBox(obj){
	var $obj;
	if(obj instanceof jQuery){
		$obj=obj;
	}else{
		$obj=$(obj);
	}
	$obj.show();
	center($obj);
	$(window).scroll(function(){
		center($obj);
	});
	$(window).resize(function(){
		center($obj);
	});
}

function center($obj){
	var windowWidth = document.documentElement.clientWidth; 
	var windowHeight = document.documentElement.clientHeight;
	var popupHeight = $obj.height();
	var popupWidth = $obj.width();
	$obj.css({   
		"position": "absolute",
		"top": (windowHeight-popupHeight)/2+$(document).scrollTop(),   
		"left": (windowWidth-popupWidth)/2   
	});  
}