$(document).ready(function(){  
          
            $('#ask').submit(function(){  
                $.ajax({  
                    type: "POST",  
                    url: "count.php",  
                    data:$('#ask').serialize(),
                    success: function(html){  
                        $("#result").html(html);  
                    }  
                });  
                return false;  
            });  
			});

<form id='ask'>
<input type="checkbox" name="box[]" id="box[]" value="28">
<input type="checkbox" name="box[]" id="box[]" value="15">
<input type="checkbox" name="box[]" id="box[]" value="8">
<input type="image" src="im/send.png" >
</form>

<input type="checkbox" name="box[]" class="box" value="15">

$('.box').change(function(){
    $.ajax({  
        type: "POST",  
        url: "count.php",  
        data:'selected='+$(this).val(),
        success: function(html){  
            $("#result").html(html);  
        }  
    }); 
});