I am delighted that your problem is now resolved.

Almost everyone who has tried Dx10 with Steve Parsons' fixes has reported an increase in performance and in my experience, it allows FSX to handle the CS aircraft without the artifacts and OOM errors I occasionally used to see.

I can't remember enough about XP to advise on whether or not it will support Dx10, I am assuming that your graphics card does.

Here is a link which may be of use to you.
http://www.directx10.net/archives/Directx-Download-Download-DirectX-10-for-Windows-XP.html

I can say for certain that none of Mr Parsons' fixes affect the Dx9 side of FSX and that as long as you back up your fsx.cfg file before making any of the changes recommended in the "How to" document, none of them will affect it either.

You might wish to browse some of the posts in this forum
http://forum.avsim.net/forum/569-dx-10-discussions-hints-and-help/
for further opinions

The latest fixes are here
http://library.avsim.net/search.php?SearchTerm=DX10+shader&CatID=root

and the "How to" is here
https://dl.dropbox.com/u/52676345/DX10_Fix/Notes/DX10%20Notes.pdf

Regards,
Nick