A part of the user agreement between Apple and iTunes customers is that up to five personal computers can be authorized to play the same music purchased from iTunes. The first time I bumped into this problem, after a PC died, I found that Apple wouldn’t sell me another track because I’d used up all my authorizations. Of course, it didn’t mention this until I tried to purchase.
I think at the time I actually had three computers with iTunes installed that did work. In the end, I spent some time being scolded on the phone with Apple support. Why, they asked, didn’t I de-authorize one or two of them before they expired? Then I wouldn’t have this problem, would I?
By the time it happened again, Apple had instituted a “Feedback Link" that the customer could use to meekly request that his or her authorizations be reset. A day or two later, a note would drop into the inbox saying, without fail, that the authorizations had been reset to none.
After years of having tech support deal with this, Apple finally did something about it. Here’s how to reset ALL authorizations all at once and all by yourself in iTunes 8.