I finally got tired of the eMac on my desk at work displaying an "updates available" page every day that I've been unable to do anything about because I didn't have the password to the computer. When I asked the school tech guy several months ago, he mumbled something about planning to buy password recovery software because the school was full of computers for which they'd lost the passwords.
This afternoon I Googled "Mac 'password recovery'" and found a number of suggestions for solving the problem. Ten minutes or so of trying arcane code combinations and, voila, password reset! Two years of updates were downloading as I left work.