from the help file
Update Library
The Update Library feature in Media Center updates the database to reflect any changes made to the files in the library or to the ID3 tags.
To Update your Library, go to File | Library and select “Update Library”. Then choose from the options below.
Clean Up
· Clean cover art directory (erases unused files).
· Remove files that do not exist. This will remove non-existent files from your library. If, for example, you have recently moved your files from the C:\ to the D:\ using Explorer, Media Center will still have references to those files in C:\ because it wasn’t told the files were moved. Using "remove files that do not exist" will clear them from the library. This option will not remove files on removable drives. If you keep a database of your CDs, using this option will not remove the references to those files even though they are not on your hard drive.
File Information
This setting determines how database fields such as ’Artist’, ’Album’ and ’Genre’ get updated.
· Do not update database fields. This will make no changes to your database. You can use this in conjunction with "remove files that do not exist". If all you want to do is remove the files that do not exist, but you do not want your database fields for all other files updated, then select both "Remove files that do not exist" AND "Do not update database fields".
· Update database fields from tags. Updates all of the database information for all files. This will re-create your database fields based on the files’ tag information (exception: Playlist information will not be lost). This is like removing and re-importing, except that it will retain Playlist and other information associated with the track. Backup your library before updating it.