Authorizing iTunes accounts
When you buy music or other media through iTunes, only devices authorized by your iTunes account can use them. You can authorize up to five computers to use the files you purchase. This is useful when you want to share your media with other computers in your home or with your friends.
You can only authorize or deauthorize a particular computer directly through the computer in question. This means it is important to deauthorize any computer you plan to sell, give away, or reformat.
If you need to deauthorize a computer and don’t have access to it anymore, the only option is to deauthorize ALL computers associated with your iTunes account on your computer. This can be done only once per year, and—for some reason—only when your account has five computers authorized.
Read more on Apple’s website