You can change the interface of Windows Media Player using skins/themes. These packages modify the GUI of the program while retaining the full functionality. Luckily, Microsoft already offers its users the possibility of downloading pre-made skins. Personally, I've used some skins from their page and they look beautiful. Keep in mind that while using skins, some features might not be seen unless activated in the settings options.
Browse to the Skins page and download the package you want. Install it and then you can activate it after the installation is finished.
To switch between skins press ALT while WMP is active, then go to View > Skin Chooser. While a skin is applied and active, press CTRL + 2 and CTRL + 1 to switch between skin and library.
Skins library: http://windows.microsoft.com/en-us/windows/windows-media-player-skins#t1=all