Single window / Multiple UIs

Question

What's the best way of approaching having a single application window, which is capable of loading multiple UIs within it?  To clarify, I'd like to have a single window with a basic menu, which, depending on the option clicked on that page, launches a completely new UI within the same window (completely replacing the menu interface).  Basically, I'd like to be able to replace the content of the menu window, with the content of new user interface, and then to be able to go back to the menu interface again later.

I'd preferably like to do this in a non-paged, non-navigated WPF app, but if it's going to be easier to make this navigationally based, then I'll consider going the XBAP method.  Is there a part of the WPF framework that will allow this kind of interaction?  I've looked at doing a content load on XAML files, and replacing the Window.Content with this, but this seems *very* sloppy and not very elegant.

Is there a better way?