could use a php include, most people just create one html document and one css document that contains the basic structure of the site, the header footer nav, any elements that will be on basically every page, then they just copy and paste that into all the pages and create the content an example is my site (click my sig).
to use a php include you would first start by creating a separate document with your nav, then on any page that you want the nav insert
CODE
<? include("nav.php"); ?>
where you want it.
Note that you would have to change your pages from .html to .php for this to work although the coding would be exactly the same.
Kevin DiGennaro