Assuming your Mac's IP is 192.168.1.103:

a) Can you see the webserver from your Windows box
(http://192.168.1.103 should give you the Apache default page)
If not, check the firewall on the Mac to be sure http/port 80 can pass through.

b) Does your includes/configure.php file say
define('HTTP_SERVER', 'http://192.168.1.103');

c) You say you can load other pages - are you sharing your disk
using Samba or are you actually using the webserver?

Good luck,
Scott