Loading external code

I have created a site using HTML, DHTML and a little Javascript. I have used an external link for the CSS and for the Javascipt that drives the DHTML navigation menu. However, every page has the same heading at the top (including the navigation system) and copyright notice at the bottom. Is it possible to have the heading and footer in separate files with some sort of link to them so I only have to change one page instead of 30+ when the details alter?

I appreciate any help in advance.

you could just create a javascript file:
document.print('my header');
then link to it as follows:
<script language="javascript" src="http://site.com/file.js"></script>