I replaced a motherboard with onboard LAN card & reinstalled XP on a machine to solve this problem, it came back a week later!
turned out McAfee was somehow creating a list of acceptable IP's and changing SME server with a new IP range meant the new server could not ping the work station even though the station was a SAMBA client!
this problem also killed VPN to the mcafee infected workstation!
Finally found the right panel in Mcafee to undo the block. No idea why the local network was blocked in Mcafee, I suspect it was some cryptic message like "unknown computer trying to access your system, allow/deny" , user clicks deny, and BAM, no ping and no VPN!