There are 4 basic ways to do it, three of which require a copy of Windows.
Bootcamp (I have set up on my Mac.),
Parallels,
Fusion (The one I use most of the time.), and
Crossover (The one that doesn't require Windows.).
Darwin is just part of the OS core (OS X is a version of Darwin.), and has nothing to do with this, and Crossover is based off of Wine. As for legality, there is nothing that makes Wine illegal. I am not sure if you can get Wine to run on a Mac, as I have never looked in to it. MacPorts is just for getting open source *nix software to run on your Mac, not Windows software.