You are running PHP in CGI mode when using it with IIS, so unfortunately, you cannot use $DOCUMENT_ROOT. The only two options I can think of is:
Create an include file that contains:
define(DOCUMENT_ROOT, "c:\inetpub\wwwroot") and then include that in all PHP files - you can then call $DOCUMENT_ROOT variable.
Trash IIS and use Apache for Windows (much easier!)
HTH
Jonathen