Are your pages written in Silverlight as well or do you only have the navigation written in Silverlight.
If you have written your pages in Silverlight, you may want to take a peak at the
animation feature[
^] that's included in silverlight.
If your pages are written in html (asp.net or whatever) you may want to use
jQuery[
^] which has excellent functionality for
animating[
^] (parts of) a webpage.