I am not sure I understand the question. It seems that you have more of a desktop/WinForms development background, is that correct?
The web-based environment forces a diffeernt programming paradigm, as it clearly separates server-side functionality from client-side functionality. There are also two very distinct environments - the server environment with its APIs and the client browser.
While ASP.NET and other technologies aim to bridge that gap, developing for the web requires to consider how browsers and the protocols they use work.
With this respect, certain features are not possible to implement at all, or in a manner logical for a desktop environment.
In the case of RadWindow, as noted in our original answer - a RadWindow cannot do more than what an IFRAME can do, as it is basically a moveable iframe
[when rendered on the client].
We hope this information will be useful to you,
the Telerik team