FlyakiteOSX is a skinning program for Windows XP that will make Windows look and act like Mac OS X.
It will transform the look of an ordinary Windows XP+ system to resemble the look of Mac OS X. The installer simply automates the process of replacing critical system files, setting registry tweaks, and installing extras such as cursors, sounds, visual styles, etc.
It may sound a bit unsafe, but it's easily removed if you don't like it. The download's Web site is also fully skinned to look like OS X, which is pretty neat, if a bit hard to use.
Free download for Windows XP.
FlyakiteOSX [via Micro Persuasion]