Here is the API that one can use to set the default playback application of Windows Vista and Windows 7:

IApplicationAssociationRegistration::SetAppAsDefaultAll Method
http://msdn.microsoft.com/en-us/library/bb776338(v=VS.85).aspx

This method takes only one parameter - a pointer to a null-terminated Unicode string that specifies the registered name of the application. In this case it will be “Windows Media Player”.