$('#log-in').click(function () {
        if (ServerModel.UserId == 0) {//User not logged in, open login dialog
            $("<div></div>")
            .addClass("dialog")
            .addClass("form-dialog")
            .attr("id", "login-dialog")
            .appendTo("body")
            .dialog({
                title: 'LOGIN',
                close: function () { $(this).remove() },
                modal: true,
                width: 323,
                resizable: false
            })
            .load(ActionUrls.LogOn);
        }
    });

@using (Ajax.BeginForm(new AjaxOptions { HttpMethod = "POST", UpdateTargetId = "login-dialog" }))
{
    //Login form fileds in here with submit button at the end
}

[HttpGet]
    public ActionResult LogOn()
    {
       return PartialView("_Logon");
    }

    [HttpPost]
    public ActionResult LogOn(LogOnModel model)
    {
        if (ModelState.IsValid)
        {
            CustomSqlMembershipProvider provider = new CustomSqlMembershipProvider();

            if (provider.ValidateUser(model.UserName, Security.ComputeHash(model.Password)))
            {
                FormsAuthentication.SetAuthCookie(model.UserName, model.RememberMe);
                return RedirectToAction("Index", "Home");
            }
            else
            {
                ModelState.AddModelError("", "The user name or password provided is incorrect.");
            }
        }

        // something failed, redisplay partial view with the model
        return PartialView("_Logon", model);
    }

...
.load(ActionUrls.LogOn, function(data) {
    if (data.redirect) {
        window.location.href = data.redirect;
    else {
        // other
    }
});

@using (Ajax.BeginForm(new AjaxOptions { HttpMethod = "POST", UpdateTargetId = "login-dialog" }))
{
    //Login form fileds in here with submit button at the end
}