(A) Mac's and PC's talk a different file sharing language. They can use TCP/IP to get messages back and forth, but they are unable to 'see' each others file systems without some extra help. Windows NT and 2000 server have apple talk services that can be enabled so that Mac's can mount specific Windows shares. There are also programs from Thursby software that can be loaded on a Mac or Windows to share files. Lastly, you can always enable an FTP server on one of your computers to trade files.
Also see http://www.macwindows.com/ for more Macintosh Windows integration information.
Also see PCMACLAN, an appletalk client for Microsoft Windows.

(A) PCMacLan is a collection of networking utilities that let Windows PC’s utilize Macintosh network resources over IP. It also allows your Windows computer to create Mac compatible network shares and printer shares. On the Mac side, it provides Windows computers with the ability to access Mac Personal File Sharing, LaserWriters, DeskWriters, AppleShare IP Servers Apple, Remote Access 3.0.

(A) The PCMACLAN Print Server allows Macs to print to any PC printer, including non-PostScript printers.