When you configure Internet Connection Sharing, it should automatically configure the network card that connects to your Windows 98 system to use the IP address 192.168.0.1. It will then automatically configure a simple DHCP service--even if you're using Windows 2000 Professional. This DHCP service will assign IP addresses in the 192.168.0.x network.
So, share the connection, allow Windows to automatically configure these things, and then change your Windows 98 box to use DHCP. Then, reboot.
This was first published in August 2001