Just a couple of thoughts.......
Can you give us an idea of what network protocols you're using? As you probably know, MS has decided not to automatically install NetBEUI with XP, although it's still available on the installation CDROM if you look carefully. They want you to use TCP/IP (which is definitely better for all of us!!).
What IP addresses are you using? Can you ping the other machines from the command prompt? And can they ping you?
EDITED to say that if you're using TCP/IP only, and the laptop can read the XP identifier then you're probably OK, and will ping fine.
Have you disabled the inbuilt firewall though?
Cheers