Windows files on a Mac Server
Is it running Mac OS X Server, or just Mac OS X client?
If running Mac OS X Server, just start up the Windows sharing daemon via Server Admin, and use Workgroup Manager to define which directories should be shared.
If you're not using Mac OS X Server you can either edit the Samba configuration files by hand or get a copy of SharePoints which puts a GUI on the process.
Either way, once the server is running it should be a simple matter of telling the Windows machine to connect to the server at your Mac's IP address, typically via something like: smb://ip.address.of.mac/sharename