"Think you're downloading a new song or video? Watch out--that file may be stuffed with pop-ups and adware.
PC World has learned that some Windows Media files on peer-to-peer networks such as Kazaa contain code that can spawn a string of pop-up ads and install adware. They look just like regular songs or short videos in Windows Media format, but launch ads instead of media clips. When we ran the files, we noted over half a dozen pop-ups, some attempts to download adware onto our test PC, and an attempt to hijack our browser's home page."