non ne ho la più pallida idea perchè non vada.
So solo che ho preso il tuo codice e va

codice:
<%@ Page Language="VB" AutoEventWireup="false" CodeFile="a.aspx.vb" Inherits="prove_a" %>

<!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 runat="server">
    <title></title>
    <script type="text/javascript" src="../js/prototype.js"></script>

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

    function caricaBox(string, IdBox) {
        var url_action = string;

        new Ajax.Request(url_action, { onComplete: showResponse });
        function showResponse(originalRequest) {
            var t = originalRequest.responseText;
            t = execJS(t);
            $(IdBox).innerHTML = t;
        }
        return false;
    }

    function execJS(t) {
        var p1 = 0, p2 = 0, p3 = 0, p4 = 0;
        p1 = t.indexOf("<" + "script", 0);
        if (p1 == -1) return t;

        p2 = t.indexOf(">", p1 + 7) + 1;
        p3 = t.indexOf("<" + "/script>", p2);
        p4 = p3 + 9;

        var c = t.substring(p2, p3);
        var s = document.createElement("script");
        s.type = "text/javascript";
        s.text = c;
        document.getElementsByTagName("head")[0].appendChild(s);

        t = t.substring(0, p1) + t.substr(p4);
        return execJS(t);

    }
// ]]>

</script>

</head>
<body>
    <form id="form1" runat="server">
    <div>
        <input id="Button1" type="button" value="button" onclick="caricaBox('?ajax_comando=1&nome=pietro','div');" />
        


        vai
        


        <div id="div"></div>
    </div>
    </form>
</body>
</html>
codice server
codice:
Option Strict On
Imports l = libreria.ModuloWeb

Partial Class prove_a
    Inherits System.Web.UI.Page

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        Dim ajax_comando As String = l.RequestParams("ajax_comando")
        If ajax_comando = "1" Then
            Me.Response.Write("Ciao, Mondo!")
            Me.Response.Write("<script type=""text/javascript"">alert(""Codice javascript inviato con ajax""); /*initialize Tab Menu*/</script>")
            Me.Response.End()
        End If
    End Sub
End Class