[DanInMa]

How about a different approach?

$(document).ready(function() {
	//$('#Loading').hide(); you can use plain ole css to hide this by default, but re add it if you prefer.

$("#User_Nick").blur(function(e){
$('#Loading').show();
$.post("check_username_availablity.php", {
		username: $(this').val(),
	}, function(response){
		$('#Info').fadeOut();
		 $('#Loading').hide();
		setTimeout("finishAjax('Info', '"+escape(response)+"')", 450);
	});
})



});

//Finish Ajax Function
function finishAjax(id, response){
 
  $('#'+id).html(unescape(response));
  $('#'+id).fadeIn(1000);
}

also you can remove onblur="return check_username();"

- also how are you showing the actual pop up?