Make UpdateProgress disappear

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VS2008

ASP.NET

Visual-Studio

Ajax

I have a button that does some excel automation. I need a wait window to pop up while code is running. Once that code is done I need the wait window to disappear and that is where I have an issue.
Here is some summary of my code:

<asp:UpdatePanel ID="upPortfolioRpts" runat="server" UpdateMode="Conditional" >
    <ContentTemplate>
        <div style="float:left;">
            <asp:UpdateProgress ID="UpdProgWait" runat="server" AssociatedUpdatePanelID="upPortfolioRpts">
                <ProgressTemplate>
        <img src="images/simple.gif" alt="progress"/>
                </ProgressTemplate>
            </asp:UpdateProgress>
        </div>

         <asp:Button ID="BtnCreateExcel" runat="server" Text="Merge Report(s) To Excel" OnClick="BtnCreateExcel_Onclick" Font-Size="X-Small"  OnClientClick="ShowProgress();"/>

        </div>

    </ContentTemplate>

        <Triggers>
            <asp:PostBackTrigger ControlID="BtnCreateExcel" />
        </Triggers>

</asp:UpdatePanel>

<br />
<br />
Javascript that shows the wait window<br />
<code>    function ShowProgress() {<br />
        document.getElementById('<% Response.Write(UpdProgWait.ClientID); %>').style.display = "inline";<br />
    } 

Everything works except I can't get the wait window to disappear once the excel automation is completed. Any suggestions?