Hmmm, could be two things:
1) You haven't given your computer a name. I'm not sure about Win98, but in Win95 the way to go about this would be Control Panel > Network > Identification (tab) and fill in "Computer Name" with something, your name for example. The process could be completely different with 98 though, however I'll never use 98 so I'll never know.
2) You haven't set the ServerName attribute in the Apache config file. Open httpd.conf and look for "ServerName" (it should be up top somewhere). It should read:
ServerName localhost
If it doesn't, change it.
But it could be many other things that I'm not aware of. AFAIK Win98 has been set up better than 95 for networking, so if neither of those suggestions are helpful, perhaps a 98 guru might help you out...
adam