codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">



<html xmlns="http://www.w3.org/1999/xhtml" >
<head><title>
	Pagina senza titolo
</title>

    <script language="javascript" type="text/javascript">
// <!CDATA[
function prova_onclick(v)
{
    if(v.value != "0")
    {
        document.getElementById("valore").value = v.value;
        v.form.submit();
    }
}

// ]]>
    </script>

</head>
<body>
    <form name="form1" method="post" action="a.aspx" id="form1">
        <select name="prova" id="prova" onchange="prova_onclick(this);">
            <option value="0"></option>
            <option value="1">Stefano</option>
            <option value="2">Carlo</option>
            <option value="3">Nicola</option>
        </select>
        <input name="valore" type="hidden" id="valore" value=""/>

    </form>
</body>
</html>