I assume you mean Apache-style virtual includes?
In your .htacces (or httpd.conf if you have access) add:
AddType text/html .htm .html
AddHandler server-parsed .html .htm .shtml
However! you may experience slow downs with this as the server must parse every single HTML file. :-(
An alternative to this is adding this line to the .htaccess:
XBitHack on
Then any page that contains to-be-parsed code, just chmod it to include execute. (chmod +x file)
To do the actual includes:
<!--#include virtual="/includes/navigation.txt" -->
I hope thats what you were actually talking about!
Email me if you need more help.