Ajax Update panel

Question

hi friends pls help me.

 Read this code..

 <asp:UpdatePanel ID="UPDownload" runat="server">
 <ContentTemplate>
 <asp:Label ID="lblMessage" CssClass="LabelDefault"  runat="server" Text="Click the link to download the Report:">
 </asp:Label>  
 <asp:LinkButton ID="lbtnDownload" runat="server"  OnClick="lbtnDownload_Click" Text="Download Excel Sheet"></asp:LinkButton> 
 </ContentTemplate>
 <Triggers>
 <asp:PostBackTrigger ControlID="lbtnDownload"/>
 </Triggers>
 </asp:UpdatePanel>

In linkbutton click event i am generation excelsheet using Response.write..its working fine..but now i need to show the  progress bar so that i used the following code

 <asp:UpdateProgress ID="UPSdownload" runat="server" AssociatedUpdatePanelID="UPDownload">
 <ProgressTemplate> 
 <asp:Image ID="Image1" runat="server" ImageUrl="~/image/ajax-loader.gif" />  
 </ProgressTemplate>
 </asp:UpdateProgress>

 but its not showing the progress bar...
 ...any one help me to show the progress bar....thanks in advance.