These tips should help you get everything working:

1. Permanently disable XP's built-in Internet Connection Firewall on
local area network connections -- it's for use only on a direct modem
connection to the Internet.  Disable and un-install all other firewall
programs while troubleshooting.  When un-installing a firewall
program, use the un-install procedure provided by the manufacturer .
Don't use Control Panel | Add or Remove Programs, which might not
completely un-install it.

For more information, see:

Windows XP Internet Connection Firewall
http://www.practicallynetworked.com/sharing/xp/ic_firewall.htm

2. Use only one protocol for File and Printer Sharing.  If the network
needs more than one protocol, unbind File and Printer Sharing from all
but one of them.  Details here:

Windows XP Network Protocols
http://www.practicallynetworked.com/sharing/xp/network_protocols.htm

3.  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

4. 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