On your machine with "iTunes", go to the "iTunes store", login with your account (note, if you are using a friend’s computer be sure to log out of their account first), select the “view account” option, then there will be an option to “deauthorize all” – select this and ALL of your authorized computers will be deauthorized.
This means that if you have to take this route, you will need to then RE-authorize each of your other machines.
A better solution – if you have the chance before your computer dies (or you recycle it/sell it/give it away) be sure to open up iTunes on that machine, connected to the Internet, and deauthorize JUST that machine.