If you want to start from scratch with adding new music or movies to the XBMC meta database, you can remove the MyMusic7.db or MyVideos34.db

In my case I had to remove it from two places

C:\Users\user1\AppData\Roaming\XBMC\userdata\Database

and in

C:\Program Files (x86)\XBMC\portable_data\userdata\Database

This was due to multiple beta installations of XBMC