The following Microsoft documents (downloads) are excellent resources for understanding and troubleshooting Windows XP file and printer sharing:
File and Printer Sharing with Microsoft Windows Troubleshooting File and Printer Sharing in Microsoft Windows XP If you're impatient, start with the second document.
While your description of the symptom (error message) is superb, you've provided no background information. For instance:
o What are you trying to do? (access shared file or printer?)
o How? (using Start-Run dialog box and UNC notation?)
o What is the relevant physical network configuration (wired or wireless?) Using peer-to-peer "workgroup" or client-server "domain"?
o What network protocol are you using? (TCP/IP, IPX/SPX, NETBUI)
o What operating systems and service packs are involved (Windows XP Home Edition, Windows XP Professional; Original, Service Pack 1, Service Pack 2)
o What type of file sharing have you selected ("Shared Documents" folder, "Simple" file sharing, or "Advanced" file sharing)
You can read the reference material, provide more information, or both. It's your choice.
