Hello! Newbie to AJAX here!

I'm getting "msxml3.dll: Unspecified Error" for the line:
if (xmlHttp.readyState == 4 || xmlHttp.status == 200)
..via my Script Debugger.

Code:
<%@ Page Language="vb" AutoEventWireup="false" Codebehind="WebForm1.aspx.vb" Inherits="AjaxTesting.WebForm1"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
    <HEAD>
        <title>WebForm1</title>
        <meta name="GENERATOR" content="Microsoft Visual Studio .NET 7.1">
        <meta name="CODE_LANGUAGE" content="Visual Basic .NET 7.1">
        <meta name="vs_defaultClientScript" content="JavaScript">
        <meta name="vs_targetSchema" content="http://schemas.microsoft.com/intellisense/ie5">
        <!-- Start script -->
        <script language="javascript"> 
var xmlHttp = createXmlHttpRequestObject(); 
function createXmlHttpRequestObject() 
{ 
  var xmlHttp; 
   if(window.ActiveXObject) 
   { 
      try 
      { 
         xmlHttp = new ActiveXObject("Microsoft.XMLHTTP"); 
      } 
      catch (e) 
      { 
         xmlHttp = false; 
      } 
   } 
   else 
   { 
      try 
      { 
         xmlHttp = new XMLHttpRequest(); 
      } 
      catch (e) 
      { 
         xmlHttp = false; 
      } 
   } 
   if (!xmlHttp) 
      alert("Error creating the XMLHttpRequest object."); 
   else 
   return xmlHttp; 
} 
function process() 
{ 
       varWBS = encodeURIComponent(document.getElementById("TextBox1").value); 
       xmlHttp.open("GET", "test.aspx?WBS=" + varWBS); 
       xmlHttp.onreadystatechange = handleServerResponse; 
       xmlHttp.send(null); 
} 
function handleServerResponse() 
{ 
   if (xmlHttp.readyState == 4 || xmlHttp.status == 200) 
   { 
     document.getElementById("show").innerHTML = xmlHttp.responseText; 
   } 
} 
        </script>
        <!-- End script -->
    </HEAD>
    <body MS_POSITIONING="GridLayout">
        <form id="Form1" method="post" runat="server">
            <asp:TextBox id="TextBox1" style="Z-INDEX: 101; LEFT: 24px; POSITION: absolute; TOP: 24px" 
            runat="server" onblur='process()'></asp:TextBox>
            <div id="show" />
        </form>
    </body>
</HTML>
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The code behind for test.aspx is:

Code:
Private Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        'Put user code to initialize the page here
        If Server.HtmlEncode(Request.QueryString("WBS")).ToString = "1.2.8.5.001" Then
            Response.Write("Please enter another WBS!")
        Else
            Response.Write("You can use this one!")
        End If
    End Sub