Xp must be correctly identifying the chip and loading modern drivers for it. Have a look in device manager and such like to dientify exactly what driver (make model, etc) is loaded. From that google to see if you can find the Win 98 drivers download from the manufacturer's site or wherever.
Then completely reformat the HD and reload Win 98. kill off all loaded win 98 drivers after startup. As you then reboot and it detects hardware use the new drivers you have downloaded.
The other option is to buy a licenced copy of Win XP. These people are probably as cheap as anywhere (and they've always been prompt delivery too)
http://www.netcomonline.co.uk/desc.php?pid=102-986