I'll handle this one...
First, you do have a 10/100 Ethernet card installed, right?
Right click Network Neighborhood on your desktop, or go into Control Panel and double click Network.
Scroll down to the TCP/IP connections in the list. One should be for Dial Up Adapter, and the other will be your 10/100 Ethernet card.
When you have identified the TCP/IP item that is your 10/100 device, doubleclick on it.
On the IP Address tab, check "Obtain IP address automatically".
You may have to enter DNS numbers if supplied by your ISP. If so, on the DNS Config tab, click Enable DNS, and enter these supplied numbers under DNS Server Search Order. As you enter them, then click add, they will appear in the window just below wher you entered them.
Click OK, and click OK.
Reboot.
You should now be online.
If not, hook up a XP machine, through the start menu, go to Show My Connections. Right click on the LAN Connection icon, and select Status. On the second tab you should be able to find the automatically detected IP, subnet mask, and other numbers for that connection. Write them down. Now hook the Win 98 machine back up, start again at the top of my post above, and enter these numbers in your Win 98 machine. After reboot, you should be online.