First, make sure they both actually have a connection to your network.
On the XP machine, right-click on the folder you want and select Sharing and Security. You should be able to find your way from there to enabling sharing.
Then on the Mac, click the Go menu in Finder and select Connect to Server. Enter "smb://your.windows.computer.ip" in the connect box (obviously replacing that text with your Windows computer's IP address). That should be it. If you need more help, just say where you're getting stuck.