One quick question first: How are these machines hooked up? If you have them hooked up directly to each other, instead of into a hub/switch, them you need a crossover patch cable.
*To test if your TCP/IP and your physical network is functioning,use the PING command from a command prompt:
PING 0.0.0.0
(Where 0.0.0.0 is the ip address of the other PC)
Ok, now here are some basic rulesfor getting your network up and running:
-Both machines must have the same workgroup name
-They won't be able to "see" each other unless one of the machines is sharing a folder or a printer.
-On Win9x machines, Client for Microsoft Networks must be installed. If it isn't, you can simply add it from the Network control panel. Add>Client>Microsoft>Client for Microsoft Windows. Also, to share a folder, open the network control panel and click on the File and Print Sharing button. There you can enable File and Print sharing.
-On WinNT, you just need to make sure that TCP/IP is configured properly and the workgroup name is the same. Keep in mind that NT WS controls access by users and groups. So, for you to access a share on the NT box make sure that you setup an account by the same name and password as you are going to be using on theWin98 machine.
Hope this helps. Post a comment if you want any more specifics, but that should get you running as long as there isn't a hardware problem going on.