I found this on the web. It works. I have installed it and makes FSX SP2 look GREAT. Here is how you do it:
First, go to start, all programs, accesories, system tools, system restore. Set up a system restore point and call it "Before Direct X 10"(this is just a safety measure, just incase it installs wrong, but it should not).
Now, ensure that your video card is a Direct X 10 enabled card. Once you have checked that, download this: http://rapidshare.com/files/163675688/DirectX10_RC2_Fix_3-Pre-Final.zip
Once there, follow the prompts to install Direct X 10 for XP! No restarting required. Just fire up FSX SP2 and notice the difference(also makes Halo 2 for PC and ShadowRun for PC work on XP, normally Vista exclusives). Of course, if you have Vista, don't bother with this. But, if you are like me, and HATE Vista, this is the way to go!!!!! ;D