In Silverlight 5 (and may be earlier), you would face a real frustrating issue when using ChildWindows…the problem is as soon as you close a ChildWindow the application would freeze. Well, the solution is pretty simple it requires you to copy-paste the following piece of code to your ChildWindow’s class:

protected override void OnClosed(EventArgs e)
   Application.Current.RootVisual.SetValue(IsEnabledProperty, true);

Code Source: Silverlight Community

While I am not the guy who found it, but I sure have used it a lot of times and trust me, it does work.

Cheers! πŸ™‚