iTunes 11 for Windows: Authorize a computer to play iTunes Store purchases
You can authorize up to five computers (Mac, 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 also authorize up to three computers to play items purchased from the Audible website (audible.com). Audible is a third-party company that sells audiobooks and other spoken programs.
You can authorize or deauthorize a computer at any time. Be sure to deauthorize a computer before you sell it or give it away.
Authorize or deauthorize a computer
Do any of the following:
Authorize a computer to play iTunes Store purchases: Choose Store > Authorize This Computer.
Deauthorize a computer: Choose Store > Deauthorize This Computer.
Deauthorize a computer to play spoken programs purchased from the Audible website: Choose Store > 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.
Sign in to the iTunes Store, choose Store > View My Apple ID, and click Deauthorize All. If you don’t see this option, it’s because you have fewer than five authorized computers.
If you have trouble deauthorizing a computer, visit the iTunes Store Authorization FAQ website.