A plug-in is a component that modifies the functionality or appearance of Windows Media Player. Visualizations and audio effects, such as normalization, are examples of plug-ins. Several plug-ins are included with the Player by default; however, you can add optional plug-ins that have been created by Microsoft or others.
You encountered this message because the Player cannot find the plug-in that you selected. This can occur if the plug-in was not added or removed from the Player properly. The Player will try to remove the missing plug-in from the menu so you will not encounter this message again. If you want to use this plug-in, you must install (add) it again.
For information about adding a plug-in, see Windows Media Player Help.