Many software products are "skinnable," i.e. they have the capability of defining a different user interface. Examples of applications that can be skinned are Windows Media Player, WinAmp, WinDVD, MusicMatch Jukebox, etc.
Essentially you can replace the standard look of an app with your own design: different buttons, different colors, and so on, just by defining a template. You can either define your own or you can usually download one someone else made, and then change the UI through the app's skin chooser.
I even found one called WindowBlinds that allows you to make your Windows desktop look like a MacOS X Aqua desktop since it replaced all the icons, buttons, menu bars, and title bars with Mac-like equivalents.
