I came across a special case of connecting 2 machines, OS X and Windows XP, to do file sharing. The Windows machine needs to be able to share its files to the OS X machine but not have internet access.
First make sure all the standard File Sharing protocols are setup on each machine. On the OS X machine make sure that Windows Sharing is turned on and a separate account for network file sharing is setup.
On the Windows side, make sure that the Windows Firewall (and/or any other firewall) allows File and Printer Sharing. Then, designate a folder to be shared. I would also create a separate account for sharing here too.
The most important part of this process is manually assigning the Windows machine a valid IP address (usually one that the router will normally handout) and subnet, but leave out the gateway address.
