Mmmm now if I understood the questions.... (How the hell did I become our companies network guy!!)
this is a pretty simple network I think. A few XP and win98 computers pluged into a switch. The only protocol installed is TCP/IP. For net access a router is plugged into the switch.
The other computers can access the new XP one no problem. As far as I can tell the network setting on the new XP computer is the same as on the other XP computers.
If all else fails, you could try imaging the harddrive of one of the other XP machines, and cloning it... then just change the computer name to make it unique on the network.
And my warmest thanks to Dell tech support who told me there was nothing wrong with the computer and that it wasn't their problem. Strange how putting in a new network card solved all the problems.