How do I deactivate a computer thats has died?
Do you mean Deauthorize... If so... read on...
If you no longer have the computer(s) you want to deauthorise,
Log in to iTunes, go to "view your account info" on the itunes store, deauthorise all five, (Please Note: this can only be done Once every 12 months) and then re-authorize your current Computer(s) one at a time.
Authorise / Deauthorise About