Network config

We have 5 pc's and a server connected to a 3com superstack
3 hub. Just got cable high speed internet and have been
unable to switch between networking with server and the
internet without going back into network properties and
changing IP etc.,on the PCs running Windows 98,I could on
the one PC running XP. I Was told in this newsgroup that I
needed a router as well, which I installed, now I am
having trouble on the PC running XP as well. Any
suggestions.
I am new to networking.