Hi Sajeev,
Unlike the standard confirm dialog, radconfirm() cannot block the execution thread - this is something that cannot be done with JavaScript and that is why a callback function is used. There is a workaround however, that allows the radwindow to mimic the blocking of the thread in certain scenarios only - buttons and links. You can examine the code in the following demo:
http://demos.telerik.com/aspnet-ajax/window/examples/browserdialogboxes/defaultcs.aspx
Best wishes,
Georgi Tunev
the Telerik team