Try using the following:
Window parentWindow = Window.GetWindow(userControlReference);
GetWindow method will walk the VisualTree for you and locate the window that is hosting your control.
You should run this code after the control has loaded (and not in the Window constructor) to prevent the
GetWindow method from returning
null. E.g. wire up an event:
this.Loaded += new RoutedEventHandler(UserControl_Loaded);