How do I 'unregister' computers in my iTunes account?

If you haven't done so in the past year, you can reset all of your authorizations by going to the iTunes Store, clicking ACCOUNT (or click your account e-mail in the upper right hand corner), login, and then click "De-authorize all computers". Remember, you can only do this one time per year. Doing this will start you back at 0 authorizations, and just authorize the computers you do use.

If you can't/don't want to do that, if your other machines are still operable, you can load the iTunes library, click STORE in the toolbar, and click "De-authorize computer" and putting in your account info. You have to do this once for each computer.

EDIT: Although I just re-read your post, you can only do the first option then if your computers are recycled. If for some reason you already did the authorization reset in the past year, you'll have to contact Apple to request a reset.

Message was edited by: AHXtreme42

Note that you will not see *Deauthorize all* until you have 5 computers authorized.
See this -> About iTunes Store authorization and deauthorization