Just a word of caution, this procedure generally works fine, but I've witnessed it fail several times as well, and not let you log in afterward. This may or may not be related to booting with the wrong OS media -- I'm not sure. What's worse, in these instances, I've gone back into the change password screen and found the option greyed out. So it has seemed in certain cases to only give me one shot to do this, and then I've been stuck.

So anyway, the moral of the story is that I'd recommend backing up your data before trying it, just to be safe. And if it doesn't work, a fresh install of the OS is always a good place to start off with a new machine.

Root user is special user in Mac OS X, has high privileges and rights to make any type of change to the system, and is different than administrator. If you forgot root password, see how to reset Root user password using different methods in Mac OS X.

How to Reset Root User Password on Mac OS X

If you're running OS X, you need to start up from a Mac OS X install disk. According to Apple, it apparently doesn't have to be the original disk that came with the computer -- just "one whose version is closest to the version of Mac OS X installed." The instructions for resetting the password can be found at http://support.apple.com/kb/HT1274 under "Resetting the original administrator account password."

this is what I have just used to recover my second hand HDD which had been password locked and I don't have the original install disks (it is afterall an ibook which is 10 years old and the disks are worth more than the ibook)