There's a GUI mod called
Flyakite that actually modifies Windows so that it looks like a Mac. There's one downside to this, though... you can't undo it once its done! That being said, it DOES look very nice, but after doing this a few times, I got tired of it and just used a theme. The theme I use is called "Panther," and mixed with a Dock program (ObjectDock, etc.; I use YZDock), you can get it to look pretty nice. Here's a shot of my screen...

EDIT: Here is an older picture I took when I was using Flyakite SP2...
