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This KnowledgeBase article describes the process in Authorizing and Deauthorizing your computer to play "Protected" AAC's (ie. AAC's bought through the iTunes Music Store, personally ripped AACs are not protected)

The process appears relatively simple - with protected songs played for the first time, requiring your Apple ID/Password to play. Subsequent plays on the same computer are otherwise seamless.

Deauthorizing your computer simply requires selecting "Deauthorize Computer" in the iTunes advanced menu, which deauthorizes all the songs on your computer.

At this point, you can presumably copy the songs to another computer and reauthorize them. Note: by default, Apple allows for a song to be authorized on three computers simultaneously.