I have had computers either break or I reloaded the operating system without de-authorizing the computers from Apple's iTunes music store. There is a limit of 5 authorized computers per account, so I was in the position of not being able to play some TV shows that I purchased. It turns out that all you need to do is visit Apple's support page for iTunes that starts the iTunes application, allowing you to de-authorize all computers. Then you can re-authorize just your currently working computers. You can de-authorize all computers once per year.