You can only link 5 computers to an iTunes account. What to do if you're over the limit?
If you're getting rid of an old computer that you use iTunes on, be sure to de-authorize the computer first. That way it won't count towards your limit of 5 computers to be used with one account.
If by chance you forget, or you've forgotten in the past, you can de-authorize all computers for your iTunes account, and then re-authorize the ones that are current. This is only available if you're at the 5-computer limit, and can only be done once each year.
Instructions for both options, as well has instructions for authorizing computers, can be found at http://support.apple.com/kb/ht1420 .
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