I am using modelpop extender in my .aspx page. i want show this model popup when page is loading but not want to showing when user enter wrong input in textbox. it will also showing modelpup when javascript validation is happeing. such as

this.Page.ClientScript.RegisterStartupScript(this.GetType(), "k1", " alert(\" Incorrect UserName OR Password \");

it will also open how can avoid it.

Default.aspx---
<asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server">
    </asp:ToolkitScriptManager>
    <asp:UpdatePanel ID="UpdatePanel1" runat="server">
        <ContentTemplate>
            <asp:Button ID="btnShowPopup" runat="server" Style="display: none" />
            <asp:ModalPopupExtender ID="ctlModalPopupExtender" runat="server" TargetControlID="btnShowPopup"
                PopupControlID="pnlpopup" CancelControlID="imgOK" BackgroundCssClass="modalBackground">
            </asp:ModalPopupExtender>
            <asp:Panel ID="pnlpopup" runat="server">
                <table cellpadding="0" cellspacing="0" style="background-color: #FCFCFC; height: 200px;
                    width: 550px; text-align: left;">
                    <tr style="background: url('../Images/bg.jpg'); color: White;">
                        <td>
                            <p>
                                New Password Policy:-</p>
                        </td>
                        <td class="ClosePopupCls">
                            <asp:ImageButton ID="imgOK" ImageUrl="~/Images/close3.png" Height="35px" Width="35px"
                                ToolTip="Close" runat="server" ImageAlign="Top" />
                        </td>
                    </tr>
                    <tr>
                        <td>
                            <p style="color: Red">
                                -Default Password: 123456<br />
                                -Password should be changed after loging through default password.<br />
                                -Password should be atleast 8 Characters and combination of Numeric, Alphabets (Caps
                                Small) and Special Characters.<br />
                                -Login will be blocked after 5 unsuccessful login attempts.
                            </p>
                        </td>
                    </tr>
                </table>
            </asp:Panel>
        </ContentTemplate>
    </asp:UpdatePanel>