It's possible to use EVR under XP - it comes with the .NET 3.0 runtime - though it's not registered by default. But it's impossible to use DXVA 2.0 in XP - it requires Vista video driver model.
There was never a spec for H.264 DXVA 1.0 decoding. Any H.264 HW acceleration support under XP was likely done using custom API hooks. Looks like there will never be DXVA support for H.264 decoding in XP and DXVA 1.0. That doesn't mean there can't be HW acceleration for H.264 in XP - it just can't be done with any public DirectX APIs.