Several things to consider
1) On the one hand, you can probably disable your onboard sound in your BIOS if you want to. Have you ever tried using your onboard sound?
2) The driver you're missing has to be added to XP SP2. However it's included if you were to upgrade to XP SP3.
> Did you know MS is dropping support for XP SP2 very soon?
> As you might want to upgrade to SP3 for many reasons in addition to your onboard sound problem
In any case, we can also load the missing driver for SP2 if you want, just let me know what you're thinking to do...