Right now it's a violation of Microsoft's liscencing crap for people to make programs that modify the behavior or look of the desktop enviroment. Or something like that.
I noticed though on a windows new site that their was a program that could modify any aspect of the Windows Desktop. You know get rid of the taskbar, create floating menus, clocks and such. You can make customized stuff using visual programming products and all that stuff.
But for the life of me I can't seem to find were I found it.
All in all, it just made you desktop look like a crappy version of X windows. The screen shot was of a floating taskbar, a happy face with Eye balls that followed the mouse, a floating clock and a couple other useless stuff.
Maybe if you can find that you could create your own custom icons and stuff to make it look like OS X, their are plenty of X windows-based themes out their you could pull icons of off.
Of course it's co$ts ya.
But other then that. No, I don't think you can't make your Desktop look OS X.