I have checked the user name availability. The problem is, even if the username is not available, the form is posting.
Edited Code:
<SCRIPT type="text/javascript">
$(document).ready(function(){
$("#emailch").change(function() { 
    var usr = $("#emailch").val();
    if(usr.length >= 3)
    {
        $("#status").html('<img src="images/loader.gif" align="absmiddle"> Checking availability...');
        $.ajax({  
            type: "POST",  
            url: "includes/check.php",  
            data: "username="+ usr,  
            success: function(msg){   
                $("#status").ajaxComplete(function(event, request, settings){ 
                    if(msg == 'OK')
                    { 
                        $("#username").removeClass('object_error'); // if necessary
                        $("#username").addClass("object_ok");
                        $(this).html(' <img src="images/tick.gif" align="absmiddle">');
                    }
                    else
                    {
                        $("#username").removeClass('object_ok'); // if necessary
                        $("#username").addClass("object_error");
                        $(this).html(msg);
                    }
                });
            }
        }); 
    }
    else
    {
        $("#status").html('<font color="red">The username should have at least <strong>3</strong> characters.</font>');
        $("#username").removeClass('object_ok'); // if necessary
        $("#username").addClass("object_error");
    }
});
});
</SCRIPT>
What I want is that, the form should not be posted if the user name is not available. I have tried by using return false; in IF CONDITION but it fails. Please suggest any alternate method. Any help will be appreciated.