Unknown? Probably microsoft too ;o) It is still a big chunk for microsoft though. Doesn't excuse nVidia though.
In my mind, Microsoft deserves a lot more of the blame than NVidia.
Ok, so nvidia's drivers presumably are crap.
OK, nvidia's drivers presumably crash.
But WHY is that crash capable of taking out the entire OS in the process?!
The fact that a bad driver can still crash the entire system is 100% microsoft's fault. No matter how badly a program behaves, it should be able to crash anything other than itself (After which the OS would presumably lose communications with it, and could initiate a re-initialization of the driver in question)