public enum Status
{
    Ok,
    Error
}

public class JsonResponse
{
    public Status Status { get; set; }
    public string Message { get; set; }
    public List<string> Errors { get; set; }
}

[HttpPost]
public ActionResult Login(UserLoginModel model)
{
    JsonResponse res = new JsonResponse();

    if (!ModelState.IsValid)
    {
        res.Status = Status.Error;
        res.Errors = GetModelStateErrorsAsString(this.ModelState);
        res.Message = "Oh dear, what have you done. Check the list of errors dude!";
    }
    else
    {
        // Save it here...

        // Return success
        res.Status = Status.Ok;
        res.Message = "Everything was hunky dory";
    }

    return Json(res);
}

private List<string> GetModelStateErrorsAsString(ModelStateDictionary state)
{
    List<string> errors = new List<string>();

    foreach (var key in ModelState.Keys)
    {
        var error = ModelState[key].Errors.FirstOrDefault();
        if (error != null)
        {
            errors.Add(error.ErrorMessage);
        }
    }

    return errors;
}

<script type="text/javascript">
$("form").submit(function (evt) {
    // validate
    $('form').valid();

    // extract values to submit
    var form = $(this),
        username = form.find("[name=Username]").val(),
        password = form.find("[name=Password]").val(),
        json = JSON.stringify({
            Username: username,
            Password: password
        });

    $.ajax({
        url: form.attr("action"),
        type: 'POST',
        contentType: 'application/json; charset=utf-8',
        dataType: 'json',
        data: json,
        success: function (result) {
            alert(result.Message);
        }
    });

    // stop form submitting
    evt.preventDefault();
});
</script>

private Dictionary<string, string> GetModelStateErrorsAsString(ModelStateDictionary state)
    {
        Dictionary<string, string> errors = new Dictionary<string, string>();
        foreach (var key in ModelState.Keys)
        {
            var error = ModelState[key].Errors.FirstOrDefault();
            if (error != null)
            {
                errors.Add(key, error.ErrorMessage);
            }
        }
        return errors;
    }