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 five computers at once: Macs, PCs, or any combination. You just need to type in your Apple user name and password on each computer to authorize it to play any songs purchased with that account. Each computer must make an Internet connection to relay the information back to Music Store headquarters. (And don’t worry—you don’t have to authorize each and every song because you authorize the computer to play all the songs purchased from an account.)
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.
Method 2: Just drag the song you want out of the iTunes ...