I had purchases from the iTunes Store authorized on three of my computers — one personal computer and two machines at work. The office I.T. guys swapped out my work computers but I forgot to deauthorize iTunes on them before they wiped the hard drives clean. Now what, since Apple only lets me authorize a few computers?
Apple lets you “authorize” up to five computers at a time to play copy-protected music, movies and other content purchased with your iTunes Store account. (Music in the iTunes Plus format does not contain digital-rights restrictions, but songs purchased before early 2009 and most commercial videos have it built in.)
So what do you do when you hit that five-computer limit and you no longer have access to all of your authorized machines? Start by logging into your iTunes Store account on one of the computers you can still get to; click on the Account link on the iTunes Store home page to get there.
In the account sign-in box, enter your iTunes Store user name and password and click on the Account Info (or View Account) button. The top part of the Account Information screen lists the number of computer authorizations you have made. When you have five computers authorized, you should see a Deauthorize All button. Click the button to deauthorize all your computers at once. (Apple notes you can only use the Deauthorize All button once a year.)
Now that you have deauthorized all your machines, you can re-authorize each one you still have by clicking on a copy-protected purchase in your iTunes library and entering your iTunes Store name and password. Apple has more information on iTunes Store authorizations here.