I cant answer directly to your problem but I solved a similir one. most XP machines allocate an address TCP/IP automatically and any firewall should be able to cope but on my network there were various firewalls that could not cope with dynalmic allocation of TCP/IP and so the solution was to ensure that all the networked machines had a static allocated IP address and thuis the firewalls were then configured to allow access to thoise machines.
The internet actually worked too, as previously it was not fully opertaional due to those firewalls.
lots of luck
M