Preventing Psot till after validation

Preventing Psot till after validation

Hello,
I have an extra button on my page that uses the Axax method to check if a username is taken(This works).
But, I dont want it to submit unless there are no errors on the validate method. Not sure where to put them.
$(document).ready(function()
{$('.error').hide();
$("#CreateAccount").validate(
{
    errorPlacement: function(error, element) {
     error.insertAfter("#chkAcct" );
   },





   rules: {
     Username: {
       required: true,
       minlength: 3
     }
  },
   messages: {
     Username: {
       required: "",
       minlength: jQuery.format("At least {0} characters required!")
     }
   }
});
$('#chkAcct').click(function(){
    var name = $('#Username').val();
    var data = 'Username='+name;
    $.ajax({
        type:"POST",
        url:"CheckName.php",
        data:data,
        success:function(html){
            $(".error").show();
            $(".error").text(html);
        }
        });
        return false;
    });
});


























 
Thanks