you need vista for true dx 10. xp does not support dx10 at all.
there are, however, patches for XP that will give you "dx 10 graphics" using dx 9. they look nice, but arent as good as true dx10.
if this is for the computer in your sig, i dont think it could even run crysis on low settings with dx10, nevermind ultra high, which is the only place you would notice the difference between dx9 and dx10 anyway