A few people have asked for code similar to this. Basically this will allow you to show/hide layers where one layer hides and another shows in it's place depending on what you click. This code comes with Dreamweaver MX, and if you have it and know how to use it – it's nice there because it gives you a UI to work with, but the same effect can be accomplished by hand.
Rather than post all the instructions here, I've put the information in the demo file(s) which you can download here.
Here's basically what it does:
show layer 1
show layer 2
hide the layers