Make sure all of your computers are in the *SAME* Workgroup.
For "File Sharing", basically all you need to do is to right click those folders you want to have available over the network and select the "Sharing and Security" option. Once "shared" their icon will have a hand under the icon. For "Printer Sharing" have a read of this - click here
Any firewall software will need to be configured to allow access to the networked computers, which may involve adding their IP addresses in a "trusted" section.
XP does have a Networking wizard which you might find helpful, but I've never used it and find some of its options confusing.