The step you're missing is to enable file sharing on the computers. You do this in Sharing Preferences on the Macs and on the PCs you have to specify at least one folder to share (you may also have to deal with XP Home's overly complex sharing protection, but that's something that's only a bit more involved). In XP you right-click on a folder, choose "Sharing" and enable it. You can also specify a sharing name for the folder that's different from its regular name. No biggie, really. Google "XP sharing" and you'll get a ton of good instructions-the first hit I got was straight from Microsoft.
Enjoy!