Irony
I do this partly because I like it, partly because I'm obnoxious, partly because the only reason I have a windows box is I could afford to put one together for a few hundred bucks from parts, and would rather have a Mac but can't afford one yet.
Oh, and I do this partly rebellion against Microsoft for making that atrociously ugly 'Chiclets' interface standard on XP. Aqua has style and looks like it was designed to be pretty. The XP Default theme looks like it was designed to appeal to three-year-olds. Yuck.
Ironically, (with the exception of the occasional odd Tk-based Linux app port and stuff like that), there's ONLY one application which absolutely will not take the skins, overriding any attempt at skinning with its own, and this depriving me of the pretty stoplight buttons and leaving me with only the old _ [] X buttons instead...
You guessed it.
iTunes.
DIY Laptop, Windows XP Pro