a driver translates between software and hardware.
windows basically tells the driver something like “I would like to play this and that with these options” and the drivers translates this into commands that makes the soundcard play it.
there are big differences between win98 and xp drivers, I cant explain you what differences, but 98 drivers are simply useless in XP. (its like a completely different language).
and developing your own drivers … I dont think so. You either need complete knowledge and understanding of the hardware so you can write your own driver, or you need alot of brain to reverse-process the win98 drivers and then rewrite it for XP (which means you basically have to completely understand what the driver does), but this does also require alot of knowledge of Win98 and WinXP and the card.