Q: How can I find out which computers I have authorized for my iTunes Store account? Can I deauthorize computers that I no longer have?
A: There is no way to actually identify which computers you have authorized as the iTunes Store does not provide this information. Further, you can only de-authorize individual computers from iTunes running on those computers, so there is no easy way to individually deauthorize a computer that is no longer in your possession, or one that has had the operating system reinstalled.
The good news is that the iTunes Store does permit you to reset your authorizations to zero once per year, but only after you have reached your five-computer limit. Once you have five authorized computers, when you log into your iTunes Store account profile page, you should see a “Deauthorize All” button appear beside the number of computers you have authorized.
Clicking this button will reset your authorization count to zero and you will then need to manually reauthorize any computers that you are still using with iTunes.