DirectX 9.0c February 2007 Update is the latest for Windows XP
DirectX 10.0 & 9.0Ex February 2007 Update is the lastest for Windows Vista
Windows XP is incapable of running DirectX10 because it requires you're graphics card to be using the new Windows Driver Model which is currently only supported in Windows Vista, as it was changed from a HAL (Hardware Abstraction Layer) to Direct Hardware. The entire output model means that without some very heavy modification it's impossible to install and use on the Windows XP platform.
It'd be like trying to run DirectX on Linux by just adding support for Win32 Libraries and expecting the system to know what to do, just can't happen.