Maximum number of authorizations - how to de-authorize?

Question

I have been legally purchasing songs on iTunes for many years and on more than 5 computers.  Many of those computers were at studios where I worked and are now at the bottom of landfills.  I have two laptops, one of which states that it can't play certain songs I purchased years ago becuase "the maximum number of computers authorized" was reached.  The number being 5.
How do I deactivate a computer that is at the bottom of a landfill?  And thus be able to play or transfer the 100's of songs on my old laptop that Apple says I can't play, even though I legally purchased them?
Newer laptop is:
Macbook Pro
OS 10.6.8
iTunes 10.6.3
Older laptop is:
Powerbook G4
OS 10.5.4
iTunes 8.x

Answer 1

For computers: You can de-authorize individual computers, but only by using those computers. Otherwise you can "deauthorize all" from your iTunes account.
Open iTunes on a computer
From the Store menu, select "View my Account..."
Sign in with your Apple ID and password.
Under "Computer Authorizations" select "Deauthorize All".
Authorize each computer you still have, as you may require.
You may only do this once per year.
After you "deauthorize all" your authorized computers, authorize each one as required.
It is also possible to have mistakenly authorized a computer more than once. To fix that:
Open iTunes.
From the Store menu, choose "Deauthorize This Computer…"
Enter your account name and password.
Click OK. A message appears reading "This computer has been successfully deauthorized."
Click OK.
Repeat steps 2 through 5 above. If you see the same message reading: "This computer has been successfully deauthorized," then your computer was using multiple authorizations.
Repeat steps 2 through 5 above until you see a message reading "This computer was not authorized. To authorize this computer, play a song or video you have purchased using this account."