The Windows API!!!
Just off hand that doesn't make any sense to me. Could there possibly be a Windows API? Windows is a large and complicated system not just a group of software defined interfaces. I'm not a programmer (at least not a real one) so go gently on me if I'm way off base here.
We are in the realm of speculation now. Some form of hardware virtualization must be implemented in addition to the high-level APIs in order to fully emulate Windows. But it seems feasible for a well-funded company like Apple to do it.