used Ajax Update Panel then took upload control and show button. data added in the grid but every time refresh the page how can i prevent the refresh?

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ASP.NET

Hi All,
used Ajax Update Panel then took upload control and show button. data added in the grid but every time refresh the page how can i prevent the refresh?

plz ans me ASAP.U will be highly appreciated.

Code is:

Partial Class ctlFileUpload
    Inherits System.Web.UI.UserControl
    Dim dtFiles As DataTable = Nothing

    Protected Sub btnShow_Click(sender As Object, e As System.EventArgs) Handles btnShow.Click

        If fUpLoad.HasFile Then
            Dim filename As String = Path.GetFileName(fUpLoad.FileName)

            'fUpLoad.SaveAs(Server.MapPath("~/Files/") & filename)
            lblDataUpload.Text = "Upload status: File uploaded!"

            Dim dr As DataRow = dtFiles.NewRow()
            dr.BeginEdit()
            dr(0) = filename
            dr(1) = DateTime.Now.ToString()
            dr.EndEdit()
            dtFiles.Rows.Add(dr)
            ViewState("dtFiles") = dtFiles

            gvShowData.DataSource = dtFiles
            gvShowData.DataBind()

        End If
    End Sub

    Protected Sub Page_Load(sender As Object, e As System.EventArgs) Handles Me.Load
        If Not IsPostBack Then
            dtFiles = New DataTable()
            dtFiles.Columns.Add("FileName")
            dtFiles.Columns.Add("FileTime")
            ViewState.Add("dtFiles", DirectCast(dtFiles, Object))
            gvShowData.DataSource = dtFiles
            gvShowData.DataBind()
        Else
            dtFiles = DirectCast(ViewState("dtFiles"), DataTable)
        End If
    End Sub

    End Class

<%@ Control Language="VB" AutoEventWireup="false" CodeFile="ctlFileUpload.ascx.vb" Inherits="ctlFileUpload" %>
<asp:ScriptManager ID="ScriptManager1" runat="server" EnablePartialRendering="false"  />
<style type="text/css">
    .style1
    {
        width: 70%;
        border-style: solid;
        border-width: 5px;
    }
    .style2
    {
        height: 32px;
    }
</style>
<asp:UpdatePanel ID="upDatePanel" runat="server" UpdateMode="Conditional">

    <ContentTemplate>
        <table align="center" cellpadding="3" cellspacing="4" class="style1">
            <tr>
                <td class="style2">
                    <asp:FileUpload ID="fUpLoad" runat="server" />
                </td>
                <td class="style2">
                    </td>
            </tr>
            <tr>
                <td>
                    <asp:Button ID="btnShow" runat="server" Text="Show" />
                </td>
                <td>
                    &nbsp;</td>
            </tr>
            <tr>
                <td>
                    <asp:Label ID="lblDataUpload" runat="server"></asp:Label>
                </td>
                <td>
                    &nbsp;</td>
            </tr>
            <tr>
                <td>
                    &nbsp;</td>
                <td>
                    &nbsp;</td>
            </tr>
            <tr>
                <td rowspan="3">
                    <asp:GridView ID="gvShowData" runat="server">
                    </asp:GridView>
                </td>
                <td>
                    &nbsp;</td>
            </tr>
            <tr>
                <td>
                    &nbsp;</td>
            </tr>
            <tr>
                <td>
                    &nbsp;</td>
            </tr>
        </table>
    </ContentTemplate>
 </asp:UpdatePanel>