One of the features of Windows 7 is DirectX 11, which is a superset of DirectX 10.1 already used in Vista Service Pack 2.
Microsoft had promised that Vista users wouldn't be left in the cold, and would be able to download and use DirectX 11. It included DirectX 11 in a beta version of its "Platform Update" available in September. The final version is now available via Windows Update.
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DirectX 11 Now Available for Windows Vista - KB971644
