As you know, the iTunes Music store gives you a lot more freedom to use your downloaded songs than other music services. There are a few restrictions, though.
You can play Music Store–bought songs only on an authorized computer. Authorization is Apple’s copy protection scheme.
Between work, home, and the family network, not everyone spends time on just one computer these days. So Apple lets you play your Music Store songs on up to three computers at once: Macs, PCs, or any combination. You just need to type in your Apple user name and password on each computer. Each must make an Internet connection to relay the information back to Music Store headquarters.
You authorized your first machine when you signed up for an Apple Account for the iTunes store.
To authorize a song to play on another computer, follow these steps:
Find the song you want to transfer.
This step, of course, involves finding the song on your hard drive.
Method 1: On the Mac, open your Home→Music→iTunes→iTunes Music folder; in Windows, open My Documents→My Music→iTunes→iTunes Music. Music Store files are easily recognizable by their .m4p file name extensions.