I'm running php on the same platform. I have no value set for include_path either. The files I include are kept in a folder named /lib/ under the same folder as /bin/ where my scripts are kept. When I include an .inc file for a script in the /bin/ directory, I use the statement:
require '../lib/FileName.inc';
This has always worked fine for me.
I don't think it matters but my php.ini file is in c:\WINNT\ rather than c:\WINNT\system32. Good Luck