example
<asp:scriptmanager id="ScriptManager1" runat="server">
</asp:scriptmanager>
<asp:button id="Button1" runat="server" text="Button" />
<cc1:modalpopupextender id="ModalPopupExtender1" runat="server"
cancelcontrolid="btnCancel" okcontrolid="btnOkay"
targetcontrolid="Button1" popupcontrolid="Panel1"
popupdraghandlecontrolid="PopupHeader" drag="true"
backgroundcssclass="ModalPopupBG">
</cc1:modalpopupextender>
<asp:panel id="Panel1" style="display: none" runat="server">
<div class="HellowWorldPopup">
<div class="PopupHeaderClasss" id="PopupHeader">Header</div>
<div class="PopupBody">
<p>This is a simple modal dialog</p>
</div>
<div class="Controls">
<input id="btnOkay" type="button" value="Done" />
<input id="btnCancel" type="button" value="Cancel" />
</div>
</div>
</asp:panel>
<style>
.PopupHeaderClasss{ padding: 0.3em 0.1em; text-align: left; border:solid 1px black; background-color: #5871c3; font-weight:bold; }
</style>