No idea how it might work. But if its anything like the TPM stuff the compewter would hold some chip with keys you could not access except by running the funcitonality stored on the chip that has access to the internal keys. You then use that to decrypt the video. Which would have to be encoded to your key. Perhaps it has some specific key to communicate with MS, who uses the chip public key to send you some AES type symmetric key with the video. Then you could only look at it with the compewter that has the chip. Of course, once your compewter blows you could forget the videos, or watching them on any other system. Unless you could register several and the video would be encoded for all those keys.
Just making stuff up here, not that I know..