Q: How can I clear a WMP Library?
ANS: Delete old library database file and create a new library database file.
How can I create a new library?
1. Close Windows Media Player.
2. Locate the database file on the computer that contains your library entries. Typically, the library database file (CurrentDatabase_219.wmdb) is located in the following directory example:
a. C:\Documents and Settings\Jerry\Local Settings\Application Data\Microsoft\Media Player
3. Delete the library database file, i.e., *.wmdb
4. Start the Player. When you do so, a new library database file is created.
5. Press F3 on your keyboard to open the Add to Library by Searching Computer dialog box.
6. Click the Browse button to locate a folder on your computer that contains digital media files. Specify the folder location in the Look in box.
7. Click the Search button.
a. The Player will search for digital media files and playlists in the folder that you specify and add them to your library. If you have your files and playlists stored in different folders on your computer, you may need to search for files several times, specifying a different folder each time.
Source: http://www.microsoft.com/windows/windowsmedia/knowledgecenter/mediaadvice/0089.mspx