The basics for troubleshooting are:
1. Make sure you have the same IP scheme (e.g. 192.168.0.x).
2. Make sure you have the same Subnet Mask, Gateway, and DNS IP addresses.
3. Make sure you can ping the other computer's IP address. If this fails, just for testing, make sure you turn off XP's as well as any other firewall.
4. Make sure you can ping the other computers by name.
5. Make sure you have the same workgroup name (watch for trailing spaces)
6. 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.
7. 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.
8. More details about how to network XP can be found at:
9. More details about how to troubleshoot TCP/IP networks can be found at:
10. Apply the registry edit to fix the browsing delay from XP to Win9x computers
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 works fine so there is no need to change that either.