Ajax ModalPopup wrong display in IE8 - IE9

I'm using ModalPopup (AJAX) in my aspx web (VB.NET) using the master page, it works fine (ie6, Firefox), but when I use IE7, IE8, IE9 when I want to open Modalpopup, (the screen is enlarged and when I use the mouse wheel the screen gets bigger and bigger :() I am already trying to change the doctype by adding '<meta http-equiv="X-UA-Compatible" content="IE=8".

Hope you can help me ...

* Im using AJAX TOOLKIT 3.5, VISUAL STUDIO 2008

Here is my code:

ModalPopUpPage



<asp:ModalPopupExtender ID="Panel7_ModalPopupExtender" runat="server"                                                              
        CacheDynamicResults="True" 
        DynamicServicePath="" Enabled="True"                                                                     
        PopupControlID="Panel7" CancelControlID="Button6"
        TargetControlID="label3" OkControlID="label3" 
        RepositionMode="None" >
</asp:ModalPopupExtender>
 <asp:Panel ID="Panel7" runat="server" BackColor="#EAEFF5" BorderStyle="Solid" 
        Width="100%" CssClass="" >
        <asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Always">
        <ContentTemplate>
 <asp:GridView ID="GridViewBusAna" runat="server" AllowPaging="True" 
                AutoGenerateColumns="False" CssClass="tab" PageSize="6">
                <Columns>
                    <asp:CommandField ShowSelectButton="True" />
                    <asp:BoundField DataField="pdcNro" HeaderText="" />
                    <asp:BoundField DataField="pdc" 
                        HeaderText="" />
                    <asp:BoundField DataField="" HeaderText="" />
                </Columns>
                <SelectedRowStyle BackColor="#CCCCCC" />
            </asp:GridView>
            <br />
        </ContentTemplate>
        </asp:UpdatePanel>
    </asp:Panel>