Disable a page dynamically

[helmut]

I've an aspx page and I want to disable that page dynamically on a IF condition.Here, By the word 'Disable' I mean an exactly same condition when a pop-up or a Radwindow opens and the Parent page gets disabled and the user is not able to do anything to the parent page until the pop-up gets closed.For Ajax or Rad Controls, I can set the 'Modal' attribute of the control to true to make Parent page disabled. But what to do for my required condition.Any suggestion would be appreciated.