What are the default Global Hotkeys for Windows Media Player?
Windows Media Player features some global keys that you can use while the window is activated. If you minimize it, the shortcuts won't work. You can see the official shortcuts using the link below. Additionally, a plug-in can be installed that extends the global hotkeys even when the window is minimized.
Official keys: http://windows.microsoft.com/en-us/windows/media-player-keyboard-shortcuts#1TC=windows-7
Plug-in page: http://wmpkeys.sourceforge.net/
Download and install the file, then go to Organize > Options > Plug-ins and activate WMPKeys. Click on properties to assign shortcuts that are globally available.