You bought a directx11 card for XP? they are Dx9 compatible I guess.
You firstly need to clarify your xp build.. is it XP Home or Media Center Edition, as there is no such thing as Home Media edition. It is unlikely that it is the media center edition as that has ended its support cycle and is no longer deployed on new systems(unless its a rig you built yourself of course).
Once you have determined your version of windows, go and get the latest 10.5 catalyst drivers from this link.
These were updated just a few days ago.
The latest directX was updated in February this year
, please ensure you had that.
*make sure XP is updated to Service Pack 3
>> important seeing you have a newer card
Also, a fresh install of the game wouldnt hurt after ensuring you have all these updates. Try running the game without hacks/mods/no-cd fixes etc, off the original disc of course
You mentioned - onboard sound - it is important you determine your sound device and get the latest drivers for that.
You can use your device manager in windows to identify the model, if not the simple program PC wizard
will give you a nice list of your specs and we can point you to the latest drivers for your sound chip