iTunes 10 for Mac: Authorize a computer to play iTunes Store purchases
You can have up to five authorized computers (Macintosh, Windows, or both) at a time. To play a purchased item on a sixth computer, you need to deauthorize another one. An iPod, iPhone, iPad, or Apple TV doesn’t count as a computer.
You can authorize or deauthorize a computer at any time. Be sure to deauthorize a computer before you sell it or give it away.
Here are ways to authorize or deauthorize a computer:
To authorize a computer to play iTunes Store purchases, choose Store > Authorize Computer.
To deauthorize a computer, choose Store > Deauthorize Computer.
To deauthorize a computer to play spoken programs purchased from the Audible website, choose Advanced > Deauthorize Audible Account.
If you forgot to deauthorize a computer you no longer own, and you have five authorized computers, you can deauthorize all your authorized computers at once. You can use this method once a year.
Choose Store > View My Account, and click Deauthorize All. If you don’t see this option, it’s because you don’t have five authorized computers.
If you have trouble deauthorizing a computer, visit the iTunes Store Authorization FAQ website.