If you want to backup your Winamp song ratings and playcounts, as well as the playlists you create within the Media Library, this is how you can do it. You can back it up so you don't lose the information when you get a new computer and reinstall Winamp onto it.
I got this information from here and here
The files are in this directory:
%appdata%\Winamp\Plugins\ml(On Win2k/XP, %appdata% is
C:\Documents and Settings\(username)\Application Data\
On Vista, it is
The files are:
main.dat # ratings and play counts
main.idx # ratings and play counts index file
recent.dat # play history
recent.idx # play history index file
*.m3u8 # playlists stored in the media library
playlists.xml # list of the playlists stored in the media library
The first of the source pages also suggests some alternative strategies for saving this information.