Ford, McDonald’s and Microsoft are among the advertisers signed up to support what is thought to be the world’s largest legal music store. The service says that adverts will be nonintrusive and will not appear each time a song is played. As with iTunes, customers will have to download Qtrax software. They will own the songs permanently but will be encouraged to “dock” their player with the store every 30 days so it can gather information on which songs have been played.