Got my iBook back from repairs, after 3 weeks. Everything's working fine, but in revenge for not telling them the password they removed my user from the NetInfo database so I coudn't log in. No problem, I thought, just call the support line. Half an album of pop-punk later I get through, and find that in order for them to tell me how to reset the root password, I have to *fax* a proof of purchase (one of 3 vouchers that come with the computer, supposedly for buying upgrades) then ring them back (to deter thieves, you see). It's raining and the Post Office is half a mile away, so that's not happening.
Turns out this is easy, and you don't even need to know the old password...
OS X: To boot into single-user mode in OS X, hold down Apple-S while starting up. This gives you a command line where you mount the hard drive with write access. Then start the network services with SystemStarter, and use 'passwd root' to set a new root password.
or OS X: Boot up from your OS X install CD by holding down C while starting up, then in the Installer menu choose Reset Password. Set the root password, then restart.
Red Hat Linux: Boot from the installation CD, then get to a command line with 'linux rescue'. Mount the filesystem with write access, then use 'passwd root' to set a new root password.
Windows 98: When the user/password box appears, press cancel :)
Windows 2000/NT/XP: Boot into Safe Mode by holding down F8 while starting up. Login as Administrator (might need to press Ctrl-Alt-Del to get the login screen). The password is blank by default.
or Windows 2000/NT/XP: Create a boot disk (floppy or CD) and use it to reset any password.