how can i deauthorize a pc that no longer works?

Use the Deauthorize All command documented in this article, or if it isn't an option, the 'contact iTunes Store support' link at the bottom of it.

(122965)

THIS DOESN"T WORK!

I have NEVER used the "deauthorize all computers" feature ever, and now I have 5 dead computers that didn't have any psychic features installed on them beforehand to let me know when they were going to be unbootable, and so they could not be deauthorized. before they died.

The deauth all feature is telling me I can't do it for several more months, but I HAVE NEVER USED IT BEFORE EVER.

This is something that could cause violent reprisals, and so I am quite thankful that I am NOT a violent person.

Whoever programmed punitive and nonsensical features like these into iTunes that are so stubbornly non-redundant, are incredibly short sighted morons.