Win 98 access

I have 5 XP & 2Win98 workstations on a SBS 5.5. All
workstation can connect and access the Internet. I have
removed the 4.5 server and replaced it with SBS 2000. 4
out of the 5 XP workstations work fine and can surf. But 1
XP and both WIN98 workstation cannot access the web and
sometimes the 98 workstations will indicate no domain
server available to validate your password. From these 3
workstations I can ping the gateway and DNS, but cannot
ping any address outside. I have verified all settings and
even replaced NIC's

The strange thing is I can remove the SBS 2000 and put the
SBS 5.5 back on the network and everything works.

I know it's something simple. Please advise.

Thanks