Easy in Win XP simple peer to peer.
if you can already Ping the other systems all you need to do is right click the Hard disk, and click Sharing. click the link that says something about sharing the drives.
click share, and you will see the share name pop up in the text box as C, orD, or what ever drive you are sharing.
Another thing. if you can ping, but cannot see the systems on the network, adding the Netbeui protocol helps that considerably.
you can find instructions on how to add netbeui in XP
Here.
good luck, and have fun!