Complex sentence: Starting with Windows Vista, the "Platform SDK" has been renamed the Windows SDK to better reflect the content included, and to offer the documentation, samples, build environment and tools needed to develop Windows applications all in one place.
Split into simpler sentences: Starting with Windows Vista, the "Platform SDK" has been replaced by the Windows SDK. The change of name is to better reflect the content included, and to offer the documentation, samples, build environment and tools needed to develop Windows applications all in one place.