nu får du hele koden so mden kører lige nu: 
Partial Class TestReload
    Inherits System.Web.UI.Page
    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        TextBox1.Text = CInt(TextBox1.Text) + 1
        TextBox2.Text = CInt(TextBox2.Text) + 1
    End Sub
End Class
og 
<%@ Page Language="VB" AutoEventWireup="false" CodeFile="TestReload.aspx.vb" Inherits="TestReload" %>
<%@ Register Assembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
    Namespace="System.Web.UI" TagPrefix="asp" %>
<!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>Untitled Page</title>    
</head>
<body>
           <form id="form1" runat="server"> 
<script type="text/javascript">
function clickMyTrigger() {
        var myTrigger = document.getElementById("<%=Button1.ClientID%>");
        myTrigger.click();
}
</script>  
    <div>
        <asp:TextBox ID="TextBox1" runat="server">1</asp:TextBox>
        <asp:ScriptManager ID="ScriptManager1" runat="server">
        </asp:ScriptManager>    
    </div>
        <asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Conditional">
            <ContentTemplate>             
<asp:TextBox ID="TextBox2" runat="server">1</asp:TextBox>
        <asp:Button ID="Button1" runat="server" Text="Button" /><img src="Img/arrow.gif"  onload="clickMyTrigger();"/>
            </ContentTemplate>
        </asp:UpdatePanel>
    </form>
</body>
</html>