How to reset your Mac OS X password without an installer disc | MacYourself: "Tricking your Mac into creating a new user account
Power on or restart your Mac (should work for any Mac OS X system).
At the chime (or grey screen if your chime is turned off), hold down Command+S on your keyboard to enter single-user mode.
This step is optional, but it’s a good idea because it checks the consistency of the hard disk before moving on. At the prompt, type fsck -fy and press Enter/Return. Wait for the checks to complete before going to the next step.
Type mount -uw / and press Enter.
Type rm /var/db/.AppleSetupDone and press Enter.
Type shutdown -h now and press Enter.
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