Is your computer connected to a network such as a LAN or WAN? If yes, then it could be someone else's iTunes playlist you're seeing. There's also options within iTunes itself. Open iTunes, go to Edit menu, open preferences, press Sharing tab, & check the options. This setting might be the culprit also.

Keep in mind the LW playlist you see in iTunes is simply a playlist. You can play the songs but can't copy or burn them. To do that, you need to properly add them to iTunes library. ie: How to transfer music to my iPod (& iTunes)

If the playlist you're seeing is someone else's on a lan then you can only play it. To stop it showing, you need to configure it via iTunes sharing preferences options.