Not sure if I understood your problem, but you did not install DHCP server on the FOG server and you have a windows server that acts as DHCP server. Then you changed your scopes on your windows DHCP server and PXE stopped working?
If so you just need to make sure your new DHCP scopes are configured on the Windows DHCP server, see this for more help on that:
[url]http://www.fogproject.org/wiki/index.php/Modifying_existing_DHCP_server_to_work_with_FOG[/url]
If you dont have dhcp server installed on your FOG server there shoudent be any dhcp config files on it.