How can I deauthorize computers I no longer have access to?

I have reached my limit of computers I can authorize (for transferring purchases, music...) but I can not access 3 of them any longer. How can I deauthorize old computers that I do not have (2 are dead) so that I can authorize my new computer? I have a 2nd gen iPod Nano and use Windows (XP & 7)

-Ed