If you are not using a server, then it will be a "peer-to-peer" network in which all computers have an equal status - there is no "host" and there are no "clients". That doesn't prevent you from storing all of the files that you want to work with in a single computer, although only one computer will be able to open a particular file at a time.
The PCs will need to be connected and, as presumably you want them to have internet access, this will be via a "Router" to which they can connect by ethernet cable or "wirelessly".
If you are running XP *Professional*, then to access a remote computer, you will have to be logged in to the local computer with a Username/Password that matches a User Account set up in the Remote computer. More about this at click here
Any folders that you want to have available over the network should be "shared" by right clicking and selecting the Sharing & Security option. They should then be displayed in "My Network Places" in each of the networked computers.