How to de-authorize a computer you no longer have from iTunes?
You can find the details on apple’s website at: support page on deauthorization.
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.