cannot ping to only two computers in LAN

Dear All,
I have a lan with 20 computers. I have problem with two computers A & B. I can ping to server, router, any other computers from A but not to computer B. Similarly I can ping from B to all except A. All computers can access internet.
A & B has XP, symantec endpoint protection installed on it. Server also has got symantec endpoint protection manager.
I tried to ping disabling Firewall and endpoint protection in A & B.
Pls help me out.
Thanks,
Suja