Make sure all the computers belong in the same Workgroup. On XP, rightclick my computer > properties, computer name tab, edit the name you want on your computer create a workgroup name.
(you can name it Ranch or anything you like, just as long as all the computers belong on the same Workgroup)
Do this on all your computers. (Vista is a little different, but renaming computer and workgroup is also in the computer properties)
After the computers belong to the same workgroup, rightclick on a file you want to share and click sharing and security.
Check box share this folder on the network. Also I recommend you clicking on "learn more about sharing"
Make sure you specify correct permissions, because if anybody else is on your network, they will be able to view/modify your files.
If you did this correct, go to my network places and you should see computer icons with the names of the computers. double click on the computer you enabled the folder share to access the folder.
Make sure you did the permissions correctly, you don't unauthorized people viewing/deleting your files.
Hope this helps.