to use Unix and add a modified version of something else on top like Apple did
Perhaps they should... If MS could use their inside knowlage to create a working stable version of a windows kernel that could run on a nix platform, seamlessly like Wine's compatability layer does (but one that works well), then they could still start fresh, with a spanking new unix based system and contain the entire legacy within a seamless emulation layer or slave kernel.
UNIX is the "big daddy" OS whose legacy spawned generations of new and improved ideas, the Windows legacy is like a huge turd that wont flush, blocking the innovation pipe.
It simply had better standards for the way it works from the start. MSs kernels from inception have been inferior in critical areas like reliability, stability, in later versions multitasking, task management, networking and proper file permissions (non existant in windows - UAC in vista is actually their first serious attempt at a "workaround" for this old problem).
Unix Spawned: LINUX, Solaris, OSX, BSD, AIX, HP-UX, UltriX, etc...............
DOS/Windows Spawned: ..Windows. (& A few incomplete opensource compatability layers made & maintained by people desparate to break away from it!)
If you cant beat them (performance wise, rather than with the not-so-bottomless wallet) then perhaps its time to join them.