<li class="loginlink">
    <a id="showlogin" href="#">
        <span style="color: #666666">Login</span>
    </a>
    <div class="loginbox" style="display: block;">
        <fieldset>
            <label>User Name : </label>
            <input id="input" type="text" value="" name="input">
        </fieldset>
        <label>
            <span style="display: inline-block; ...;"> Password :</span>
        </label>
        <input id="password_txt" type="password" style="padding:5px;..;"
               value="" name="password_txt">
        <p>
            <a class="loginlink" onclick="mojarra.jsfcljs(document.getElementById('headerForm'),
                     {'j_idt60':'j_idt60'},'');return false" style="color: #666666;.."
                     href="#">Forgot Password?
            </a>

            <a class="loginlink" onclick="mojarra.jsfcljs(document.getElementById('headerForm'),
                      {'j_idt63':'j_idt63'},'');return false" style="..."
                      href="#">Register
            </a>
        </p>
        <div class="loginbuttons">
            <input id="loginBtn" type="submit" value="Login" name="loginBtn">
            <input id="cancellogin" type="button" value="Cancel">
        </div>
    </div>
</li>

$('#showlogin').click(function(){
    var loginBox = $('.loginbox');
    loginBox.show();
    $('.loginbox fieldset input').focus();
    if (!loginBox.is(':hidden')) {
        validateUser();
    }
});

$('#cancellogin').click(function(){
    $('.loginbox').hide();
});

function validateUser() {
    $("#loginBtn").click(function(event){

        var userName =  $("#input").val();
        var password = $("#password_txt").val();

        if (userName == "") {
            $.dialog({
                message: "UserName must be entered",
                imageIcon: false,
                type: "error",
                okButtonID: "ok",
                okButtonValue: "OK"
            });
            return false;
        }
        return true;
    }); //end of click
} //end of validateUser()

function validateUser() {
    $("#loginBtn").click(function(event){
    ...
    });
}

$(document).ready(function () {

    $("#loginBtn").click(function (event) {

        var userName = $("#input").val();
        var password = $("#password_txt").val();

        if (userName == "") {
            $.dialog({
                message: "UserName must be entered",
                imageIcon: false,
                type: "error",
                okButtonID: "ok",
                okButtonValue: "OK"
            });
            return false;
        }
        return true;
    });

    $('#showlogin').click(function () {
        var loginBox = $('.loginbox');
        loginBox.show();
        $('.loginbox fieldset input').focus();
        // code to call validate user removed from here!
    });

    $('#cancellogin').click(function () {
        $('.loginbox').hide();
    });
});