Unfortunately, IIS include directive doesn't recognize root addressing nor can you dynamically try to generate a root path. My assumption is that you are trying to avoid nested include problems. The following is the best way to get around this:
You will need to create a Virtual directory in IIS and place your include files there. Then reference them as
<!--#include virtual="myincludefolder/myfile.asp"-->
Being referenced as a Virtual directory allows IIS to
always "start" from "myincludefolder" whenver the file is referenced. So you could have files that include files
and be called from a folder 6 levels deep and still
not having any problems.