What error message are you getting?
Can you see the computer on the network?
Make sure you have the same workgroup name (watch for trailing spaces)
For troubleshooting purposes, turn off XP's and completely uninstall any other firewall software. You can always add more complexity after you get it working.
With XP, make sure you have the same username and password as the person logging onto the other computers. The default setting for XP Pro is to require a password for network access.
More details about how to network XP can be found at:
http://www.onecomputerguy.com/networking/xp_network.htm
More details about how to troubleshoot TCP/IP networks can be found at:
http://www.onecomputerguy.com/networking/trouble.htm
There continues to be a lot of misinformation about needing NetBEUI or to changing the NetBIOS setting. You can ignore both. Installing NetBEUI to solve a networking problem will just mask a some underlying and potentially important misconfiguration with TCP/IP. The default NetBIOS setting usually works. If browsing is a problem, you might set it to Enabled. But do not Disable it.