If you are talking about a modal popup then this would be easily done using
Ajax Control Tool Kit
Once you installed ajax control tool kit to your tool box items, it is ready to use.
1) Now add a panel to your page designer, add an inner panel and put all the controls you want in the modal popup to that inner panel.
2) At the right end of the panel you will find a smart menu
Add Extender
. Click it and select
Modal Popup Extender
from the list.
3) Set the
Drag
property to true and set the
PopupDragHandleControlID
property to the ID in the inner panel in step 1.
4) The size of the popup will be the size you set for the panel.
5) Set the
PopupControlID
of the extender to the ID of outer panel in step 1 and the set the
TargetControlId
of the extender to the ID of a clikable control like button.
Now you can drag the popup
An Example markup here
<asp:Button ID="Button2" runat="server" Text="Button" />
<asp:Panel ID="Panel1" runat="server" >
<asp:Panel ID="Panel2" runat="server" GroupingText="Dragging Panel" >
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
<asp:Button ID="Button1" runat="server" Text="Button" />
</asp:Panel>
</asp:Panel>
<asp:ModalPopupExtender Drag="true" DropShadow="true" PopupDragHandleControlID="Panel2" ID="Panel1_ModalPopupExtender" runat="server"
DynamicServicePath="" Enabled="True" TargetControlID="Button2" PopupControlID="Panel1">
</asp:ModalPopupExtender>