As an additional info, if your workstations are in an unmanaged state as thehorse13 mentioned, you can try to copy & paste the GRC.DAT file from the server (located at \\server\vphome\) to this folder on the workstation:
C:\Documents and Settings\All Users\Application Data\Symantec\Symantec AntiVirus Corporate Edition\7.5
(the path may vary a little after the '..\Symantec\' folder...)

The workstation should pick up this file automatically and re-connect to the server, when ever it's due to check for updates, or if you simply re-start the AntiVirus service.

Hope that helps.

/ SawPer

ps. But your problem sure sounds like the XP firewall issue, try to disable it temporarily to see if it helps...