Microsoft and Valve will not cooperate in the development of hardware, but join forces in Windows 10 to prepare them for the arrival of VR games. What means stays a bit unclear. It seems likely that the cooperation is intended to ensure a smooth and user experience in VR and VR Gaming.
Valve is also working on its own VR Steam platform, but the Vive experience could also tune in to the core platform of Windows 10. Microsoft is aiming for a plug-and-play platform for VR glasses!