See this ->
About iTunes Store authorization and deauthorization
"*To deauthorize all computers associated with your account*
If you find you have reached 5 authorizations, you can reset your authorization count by clicking *Deauthorize All* in the Account Information screen.
Click iTunes Store in the menu on the left side of iTunes.
If you're not signed in to the store, click the Account button, then enter your account name and password.
Click the Account button again (your ID appears on the button), enter your password, and then click View Account.
In the Account Information window, click Deauthorize All.
Note: You may only use this feature once per year. The Deauthorize All button will not appear if you have fewer than 5 authorized computers, or if you have used this option within the last 12 months. If you need assistance on using this feature, please contact the iTunes Store support via email (
http://www.apple.com/support/itunes/ww/)."
