If your server is ASP capable, change the extension of all the pages to pagename.asp and add a line in them:
<!--#include file="PieceofWebsite.inc"-->
and put all the menu code in that file. Changing the extension won't affect the pages in any other way.

If you are using Dreamweaver to create your pages, they have something called templates where you can edit the template and every page that uses that template will get changed automatically.