ti do un esempio: prendi il codice javascript, non quello server
codice:
<%@ Page Language="C#" %>

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

<script runat="server">

    protected void Page_Load(object sender, EventArgs e)
    {
        string comando_ajax = ModuloWeb.RequestParams("comando_ajax");
        if (comando_ajax == "aggiorna_database") aggiorna_database();
    }

    void aggiorna_database()
    {
        string testo = "";
        string sql = "UPDATE CAMPI SET TESTO = ? WHERE ID = 1 ";
        OleDbConnection con = null;
        try
        {
            testo = ModuloWeb.RequestParams("testo");
            con = new OleDbConnection(MioModulo.StringaConnessioneTest);
            con.Open();
            OleDbCommand com = con.CreateCommand();
            com.CommandText = sql;
            com.Parameters.Add("testo", OleDbType.VarChar, 50).Value = ModuloWeb.StringNullToDBNull(testo);
            com.ExecuteNonQuery();
            this.Response.Write("Tabella aggiornata con successo");
        }
        catch
        {
            this.Response.Write("Errore aggiornamento tabella");
        }
        finally
        {
            if (con != null) con.Close();
        }
        Response.End();   
    }
</script>

<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <title>Pagina senza titolo</title>
    <link href="../../../stili/Styles.css" rel="stylesheet" type="text/css" />
    <script type="text/javascript" src="../../../js/sajax.js"></script>

    <script language="javascript" type="text/javascript">
// <!CDATA[

function Button1_onclick() 
{
    var parametri = "testo=" + $("Text1").value;
    sajax("?comando_ajax=aggiorna_database", onload, parametri);
    function onload()
    {
        $("div1").innerHTML = this.request.responseText;
    }

}

// ]]>
    </script>
</head>
<body>
    <form id="form1" runat="server" >
        <label for="Text1">nome: </label>
        <input id="Text1" type="text" style="border:1px black solid;" />
        <input id="Button1" type="button" value="aggiorna" onclick="Button1_onclick();"/>
        <div id="div1"></div>
    </form>
</body>
</html>

la funzione ajax è
codice:
// File JScript
function sajax(url, onload, parameters, onerror)
{
	parameters = (parameters == undefined)? "" : parameters;
	
	//creazione oggetto per richiesta web
	var objHTTP = getXMLHttp();
	objHTTP.open("post", url, true);
	objHTTP.setRequestHeader('Content-Type','application/x-www-form-urlencoded');
	objHTTP.setRequestHeader('Content-length',parameters.length);
	objHTTP.setRequestHeader('Connection', 'close');

	objHTTP.onreadystatechange = function() 
	{ 
		if (objHTTP.readyState == 4) 
		{
			if (objHTTP.status == 200 || objHTTP.status == 0)
			{
				this.request = objHTTP;
				if(onload) onload.call(this);
			}											
			else {if(onerror && typeof(onerror) == "function") {onerror(defaultError); return;}else {defaultError(); return;}}
		}
	};

    objHTTP.send(parameters);
    

    function getXMLHttp() 
    {
	    var xmlhttp = null;
    	
	    if(window.XMLHttpRequest) 
	    {
		    xmlhttp = new XMLHttpRequest(); // Gecko (Firefox, Moz), KHTML (Konqueror, Safari), Opera, Internet Explorer 7
	    } 
	    else if(window.ActiveXObject) 
	    {
		    try
		    {
			    xmlhttp = new ActiveXObject("MSXML2.XMLHTTP"); // Internet Explorer 6 
		    } 
		    catch(e) 
		    {
			    xmlhttp = new ActiveXObject("Microsoft.XMLHTTP"); // Internet Explorer 4,5 
		    }
		    } 
	    else 
	    {
		    xmlhttp = null;
	    }
	    return xmlhttp;
    };

	function defaultError()
	{
		alert("ERRORE NELLA TRASMISSIONE DATI!");
	};
}

//al posto di mettere document.getElementById("div1"), mettere $("div1")
//da prototype.js
function $() {
  var elements = new Array();

  for (var i = 0; i < arguments.length; i++) {
    var element = arguments[i];
    if (typeof element == 'string')
      element = document.getElementById(element);

    if (arguments.length == 1)
      return element;

    elements.push(element);
  }

  return elements;
}