There are some great themes for Windows Vista but one really cool theme is the Vista to Leopard transformation pack.
Download it from here.
Installing it is quite straight forward.Once the setup is downloaded,run it.
Accept the license agreement and select All the options for the best possible effects of the OS X transformation pack.
Once setup is complete,reboot Vista once.The start and shutdown screen in Vista will now be that of Mac OSX.
To apply the theme,right click on Desktop and select Personalize > Theme.
Select VistaOSX theme and apply the changes.
Notice that how the Vista wallpaper changes to that of Leopard and the dockable bar appears just like in OS X.
The Windows logo is now replaced with Apple logo (yes it is the same Microsoft Windows,just the theme has changed 🙂 ).
Enjoy.