Here are some things you will want to check expanding on what the others wrote.
1) Make sure that each computer has an individual IP Address (192.168.1.1, 192.168.1.2, 192.168.1.3, 192.168.1.4) and all have the same subnet mask 255.255.255.0
2) Check and make sure all computers are running NetBeui (not required but won't hurt). To install NetBeui on XP go here: http://support.microsoft.com/default.aspx?scid=kb;EN-US;q301041
3) Make sure all computers have unique computer names.
4) If youwant to be able to access the XP computer from the 98 machines, the XP computer must have each computer name of the other computers in its user accounts. If they aren't there the computers will not be able to access shares.
5) Definetly do ping tests from each computer (these can be done from command prompt. start->run->command).
6) Check link lights on the back of each NIC card and to the hub/switch depending.
7) Make sure all computers are on Client for Microsoft Networks.
7) If all elsefails, remove TCP/IP and let it reinstall itself by a reboot.