In WMP's library, please click Organize , and select Manage libraries -
Music . What locations are listed there? If there are any unavailable locations, remove them, and also make sure that a Default save location is set. Do the same for the other libraries (Videos, Pictures, Recorded TV).
If that doesn't help, close WMP, open Windows Explorer, right-click the
Music library, select Properties , and click Restore Defaults . After that, make sure that a save location is set (as indicated by a checkmark).