Ajax e formulário HTML

<script src="http://code.jquery.com/jquery-1.7.1.min.js"></script>

<script>
function sendData(login, pass) {
    $.ajax({
        url: "pagina.php?"+$("form").serialize(),
        success: function(data) {
            receiveData(data);
        }
    });
    return true;
}
function receiveData(resposta) {
    alert(resposta);
    document.getElementById("user").value = '';// limpa o campo
    document.getElementById("pass").value = '';// limpa o campo
    document.formLogin.usuario.focus();// seta o focus
    return true;
}
</script>

<form action="javascript:sendData();" method="post" name="formLogin" enctype="multipart/form-data">
    <span>User:<br/></span>
    <input type="text" id="user" name="user"/>

    <br/><!-- space -->

    <span>Pass:<br/></span>
    <input type="password" id="pass" name="pass"/>

    <br/><!-- space -->

    <input type="submit" name="enter" value="Enviar"/>
</form>