First, make sure that all of the computers are in the same workgroup.
For XP/Win2k Right click on My computer, click on network identification, click on properties, and make sure that workgroup is selected and that all computers have the same workgroup name.
Second, create a shared folder on the computer you'd like to share the files from. Right click on the folder you want shared, click on sharing. Click on Share this folder, and create a name. Click on the permissions button if you want any special access rights.
Then browse on any of the other computers and you should see the shared folder.
There might be some issues if you're attempting this cross OS, or using WinME.