Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        Dim crd As New Drawing.Point()
        Dim i, j, x, y As Integer

        i = Convert.ToInt32(btnShowPopup.Width.Value)
        j = Convert.ToInt32(btnShowPopup.Height.Value)
        crd = New Drawing.Point(i, j)
        x = crd.X()
        y = crd.Y()
        'Response.Write(x)
        'Response.Write(y)
        mdlPopup.X = x
        mdlPopup.Y = y
End Sub

<div style="padding-left:5px;" >
        <asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager>
        <asp:Button id="btnShowPopup" runat="server" Text="Show" OnClick="btnShowPopupClick" />
        <ajaxToolkit:ModalPopupExtender ID="mdlPopup" runat="server" TargetControlID="btnShowPopup" PopupControlID="pnl" CancelControlID="btnClose"></ajaxToolkit:ModalPopupExtender>
        <asp:Panel ID="pnl" runat="server" style="display:block; border-style:groove;" Height="95px" Width="74px">
            test
            <asp:Button ID="btnClose" runat="server" Text="Close"/>
        </asp:Panel>
    </div>