Have the navigation in a div at the top of the page (I'm assuming horizontal navigation; if it's vertical, float it to the left.)
Underneath that, put a div with nothing in it and give it an id. (div id="contents"></div>).
For each link in the navigation, prevent the default action and call an AJaX function that will "get" the associated document for that link.
<a href="javascript:void(0);" onclick="ajaxFunctionGet('index.cfm')">Home</a>
<a href="javascript:void(0);" onclick="ajaxFunctionGet('about.cfm')">About Us</a>
...
<a href="javascript:void(0);" onclick="ajaxFunctionGet('login.cfm')">Login</a>
Write a javascript function that uses AJaX to call the contents of that document. If you are using jQuery it might be something like:
function ajaxFunctionGet(doc){
$.get(doc,function(data){ document.getElementById("contents").innerHTML = data; });
}
This is way oversimplified, but you should get the idea.
^_^
