One option would be to only have one content panel.  Create each "page"
as a separate UserControl and place the rest of the content on the
Usercontrol.  Then, in response to the selection on the navbar,
instantiate and display the appropriate control and place it on the
panel.

You could also use Forms for this by setting the form's BorderStyle to
nothing and it TopLevel property to False.  Then set the Form's Parent
property to the Panel and Show the form and it will appear inside the
panel.

If you're using VS2003 (will probably work in VS2002 but I don't have a copy
to test it) then see the PanelManager Class on my site which was created
specifically for this:
http://www.dotnetrix.co.uk/custom.html

In VS2005 the class still works, but because of DesignTime Designer issues
(which MS are currently working on) the control needs to be built in a
seperate ControlLibrary solution and the control referenced via the created
dll. If the control is included as part of the solution, even if it's in a
seperate project, then the Panels Collection works Ok in the designer until
the project is rebuilt, at which time if you try to access the panels
collection you get an invalid cast exception due to the designer referencing
the pre build collection items instead of the current build items.

An alternative method, as Chris suggested, is to use UserControls and load
them dynamically at runtime.