Tip 1: Check the Basics
WinXP configures TCP/IP as its default network protocol and it's all you need for both Internet and file sharing. If your Internet connection is working, then TCP/IP is installed and working, so you can at least cross that off your troubleshooting list. However - as anyone reading this article already knows - a working TCP/IP connection does not guarantee that file sharing is alive and well.
WinXP should also install and enable Client for Microsoft Networks and File and Printer Sharing for Microsoft Networks service - both of which are required to share files and printers - on all network adapters. But you should first visit the Local Area Connection Properties (Start > Settings > Network Connections) for your network adapter (Figure 1) and make sure both are there and have their boxes checked.
Figure 1: Local Area Connection Properties
Since you've got this window open, scroll down to the Internet Protocol (TCP/IP) protocol, click the Properties button, and the Advanced button on the window that opens. Figure 2 shows where you want to check next and what it should look like. The Enable NetBIOS over TCP/IP setting is needed because NetBIOS is what Windows uses for file and printer sharing.