Its easy, connect the audio out/lineout to your PC line in and then use any software like Jet audio or Musicbox jukebox +edition to convert it into .wav or .mp3. I think it is not worth converting to .wav because it takes more space and the wav file can be as good as the input you provide and the quality of an old audio cassette can't be that good. So choose a bitrate like 44000 etc and convert to ogg vorbis or mp3.