DeAuthorize Computer

I had to do the same thing.

In iTunes, click on your account (top right corner). There's a deauthorize option there. It's very simple.

 

To deauthorize all computers associated with your account:

Click iTunes Store in the Source pane.
If you're not signed in to the store, click the Account button, then enter your account name and password.
Click the Account button again (your ID appears on the button), enter your password, and then click View Account.
In the Account Information window, click Deauthorize All.

(this only works if you are using 5/5, and Apple will only let you do it once a year)

 

There should be a "Deauthorize All" option that you can use once in a 12-month period (and as long as you don't have fewer than 5 computers authorized). You can do it from a single computer, and you don't need to have access to the old, in pieces computers. After you do this, you will of course need to reauthorize those computers that you are actually using.

ETA: Oops, MattG beat me to it while I was doublechecking my memory.