Make sure that NetBIOS over TCP/IP is enabled on all computers.
Details here:
Enable NetBIOS Over TCP/IP (NetBT)
http://www.practicallynetworked.com/sharing/troubleshoot/netbt.htm
Run "ipconfig /all" on XP and look at the "Node Type" at the beginning
of the output. If it says "Peer-to-Peer" (which should actually be
"Point-to-Point") that's the problem. It means that the computer only
uses a WINS server, which isn't available on a peer-to-peer network
for NetBIOS name resolution.
If that's the case, run the registry editor, open this key:
HLM\System\CurrentControlSet\Services\Netbt\Parameters
and delete these values if they're present:
NodeType
DhcpNodeType
Reboot, then try network access again.
If that doesn't fix it, open that registry key again, create a DWORD
value called "NodeType", and set it to 1 for "Broadcast" or 4 for
"Mixed".
For details, see these Microsoft Knowledge Base articles:
Default Node Type for Microsoft Clients
http://support.microsoft.com/default.aspx?scid=kb;en-us;160177
TCP/IP and NBT Configuration Parameters for Windows XP
http://support.microsoft.com/default.aspx?scid=kb;en-us;314053