Hi
Pls try ajax tool kit's modal popup extender.You can add checkbox inside it.
<asp:Button ID="btnId1" runat="server" Text="button" />
<ajax:ModalPopupExtender
ID="ModalPopupExtender1"
DropShadow="true"
PopupControlID="Panel1"
TargetControlID="btnId1"
BehaviorID="ModalPopupExtender1"
runat="server">
</ajax:ModalPopupExtender>
<asp:Panel ID="Panel1" runat="server" CssClass="modalPopup" >
<asp:CheckBox ID="chkasdf" runat="server" AutoPostBack="true"
oncheckedchanged="chkasdf_CheckedChanged" />
</asp:Panel>
Then inside checkbox checkchanged event u can do your stuffs
Thanks