You can install a network card inside the computer. You will need a PCI or an ISA card. (I know PCI network cards are easy to find at most of the computer shops.) See this site for how to install a card: http://h10025.www1.hp.com/ewfrf/wc/document?docname=bph07111&tmp_task=prodinfoCategory&lc=en&dlc=en&cc=us&product=93139
. However you should open your computer and check the space between the devices in the slots in the computer. There is an AGP slot with a video graphics card, 1 PCI/ISA slot with a modem (default specs), one free ISA slot and one free PCI slot. However depending on the graphics card one of these slots may be blocked by the card. You'll either need to get a card that fits the available slot or remove the modem and use that slot.
Here are some PCI 10/100 network cards: http://www.tigerdirect.com/applications/category/category_slc.asp?CatId=587&name=10-100%20Ethernet%20PCI/ISA
. Most of these are compatible with Win98. I'm not sure if there are any gigabit NIC with Win98 drivers.
Wired and wireless USB NIC adapters exist. However, you will be limited to the USB speed. If you have USB 1.1, that will be really slow. Most likely your computer will require a reboot anytime the connection is lost.
I hope this helps.
(who recently removed an internal modem to put in a PCI gigabit card in a newer desktop than yours. The procedure was straightforward and improved network performance in the household immensely. The failing on-board NIC was tying up the router for everyone.)