Note: This article was originally published in 2011. Some steps, commands, or software versions may have changed. Check the current Apple documentation for the latest information.
You can find the details on apple’s website at: (http://support.apple.com/kb/HT1420). In essence, you go into iTunes, login into your account via the iTunes store. Once in you can select the “view account” option and there if you have 5 machines you should be able to see the “deauthorize all” button. After this you’ll have to authorize your computers again but should help you get rid of old machines you’ve had authorized in the past that you no longer have / or you reinstalled.
Summary
You’ve successfully learned de-authorize a computer you no longer have from itunes?. If you run into any issues, double-check the prerequisites and ensure your Apple environment is properly configured.