I'm all for free music, but not when malware comes attached to it. The Consumerist says some Spotify Free users complained that pop up ads had the damaging software in them.
Users said the malware occurred on Mac, windows, and Linux platforms. Once Spotify Free was opened, it launched the default internet browser on the computer to different virus sites. Spotify says it identified the source and and shut it down. But, If you experience the same problem - report it right away.