DirectX 10.1, The Directx 10 extension that available download via Windows Vista Services Pack 1 Build 6001.16659. This is a huge improvement over of DirectX 10
The new features of DirectX 10.1 are :

Shading and texturing capabilities,
Anti-aliasing improvements, with tighter specifications
More information of DirectX 10.1 can be found in AMD-ATI’s whitepaper

However if you want [...]

Download DirectX 10 for Windows XP is available now. You can download it ( Directx 10 for XP ) as the alpha version is said to be working by some users. It is said to work for Windows XP and with directx 10 games and graphics card, they should be able to play directx 10 [...]