Just use Samba, already built into OS10.2.
Create the shares on the Windows PCs, then on the OSX computer hit Apple+K to bring up the connect to server dialog, then for the server address type in smb://ip-address-of-computer.
To access the OSX from Windows, make sure that Windows File Sharing turned on in the Sharing panel of the System Preferences, setup the shares on the folders you want to share, and from Windows you can either browse through Network Neighborhoods or type in the IP of the OSX computer.
Very easy and quick.