How do I deauthorize a computer that has crashed?

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You can't.  However, when you have reached five authorizations, you will have the opportunity to deauthorize all of them and then reauthorize only the ones you still have or use.
Read more: 
Authorization FAQ:  http://www.apple.com/support/itunes/store/authorization/
About iTunes Store Authorization and Deauthorization:  http://support.apple.com/kb/HT1420

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