Configure IIS to Serve Unlimited Domains from 1 Set of Files?

I am a bit new at IIS so please be patient with me and put on your kid gloves for this one. Here is what I have.

I have a project on which I developed a template web site for a national auto repair chain. When a member of this organization wants a web site I begin with adding their domain to my server via a wysiwyg interface called PLESK. PLESK takes care of all the settings on my dedicated server, including those for IIS. When a new domain is initiated there is a directory is placed in the INETPUB directory in the path /inetpub/vhosts/domainname.com/httpdocs

So this is what I've done from there and am hoping for any guidance. I have created a directory called includes under vhosts called INCLUDES. In this folder I have placed files that each site uses. Then for each domain in their respective httpdocs folders I put files whose code looks like this:



See Example Page

So my question having explained how this is working is how can I make it even better. As it is now if I want to make a change on any one of the 100+ sites have all I need to do is change one file in the INCLUDES directory and wala! BUT I think there may be a better way to doing this, esp. considering that I may want to add pages to the sites and to do so would involve going site by site and adding the file. Thanks for any help on this. Tom