yes you could but trust me once you try you will not want to do it, think of darwin as linux, it has many selections of gui's i have aready installed window maker (a gui) once, but here are a few points that i'm shure will discourae you and save you a many a hours wasted:
- can't run os x apps
- can't run win apps (obviously)
- can only run posix compliant apps and most of them you will shit your head tring to install, very few have easy instalation methods, instructions. (i installed gimp over windowmaker over Xfree86 over darwin) it runs smooth but that's about the only usefull app that uses a gui that i could find a need for)
- installing darwin and then installing Xfree86 so you can run different gui's is not an easy task on a "bareneaked" darwin installation. (i installed xfree ontop of the darwin that comes with os x so i had to boot into os x to use it, this made me feel like cheating)
-and whe you say darwin on windows that's the "comand line" darwin (well there isn't other) and you still run into all the problems described above.
- it's certainly a challenge from which you may learn a great many things, but i found out that osx is by far more advantageous to use than linux or darwin exept for the fact that it's closed source. (but that has it's advantages as well wich are off topic for me to list them here)