Server Codex Forums

There is a good Mac-PC networking tutorial at MacDevCenter. I don't believe Windows 98 comes with a Samba server, so you'll have to use it to connect to your Mac OS X machine and not the other way around.

If all you want to do is transfer the occasional file, you might want to check out Haxial's KDX application. While kind of a pain to set up initially, it has proved to be very effective at moving large files quickly between Macs and PCs.