This is from Itunes Help

Authorizing or deauthorizing a computer
You can authorize multiple computers (Macintosh, Windows, or both) to play your iTunes Music Store purchases. You can also deauthorize a computer to prevent your music or spoken word purchases from being played on that computer, or to change which computers are authorized to play your purchases (you should do this if you sell your computer and want to listen to your purchases on a new computer). You can also deauthorize a computer to prevent playing Audible spoken word content downloaded from the Audible website.


To authorize or deauthorize a computer, your computer needs to be connected to the Internet.

To authorize a computer to play songs or spoken word content purchased from the iTunes Music Store, select the item (in your library or the Purchases playlist), and click the Play button.

If the computer is already authorized to play songs purchased using your account, the song plays.


If the computer has not yet been authorized to play songs purchased using your account, you're asked to enter your ID and password. Enter the ID and password for the account with which the song was purchased.


If you've already authorized five other computers to play your music purchases, you won't be able to play the songs on this computer until you deauthorize one of the already authorized computers.

To deauthorize a computer to play songs or spoken word content purchased from the iTunes Music Store, choose Advanced > Deauthorize Account. Then choose "Deauthorize Computer for Music Store Account" and enter your Apple or AOL ID and password.

If you have trouble deauthorizing the computer, visit the Music Store billing support webpage at www.apple.com/support/itunes/authorization.html.

To deauthorize a computer to play spoken word content purchased from the Audible website, choose Advanced > Deauthorize Computer. Then choose "Deauthorize Computer for Audible Account" and click OK.

If you have authorized five computers, you can also sign into your account and deauthorize all of them at once. Sign in as you normally would, and click Deauthorize All in the Account Information window. You can only deauthorize all your computers this way once per year.