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If you're using the Mountain Lion installer, you won't see this menu item. You'll get a window prompting you to select the drive on which OS X is installed so choose the drive you want to get into and select the user whose password you want from the drop-down menu. Once it does, head up to Utilities in the menu bar and choose "Password Reset". If the computer in question doesn't have a CD drive, you can either hold "Option" at boot and choose the "Recovery partition" at startup, or put the Mountain Lion or Mavericks installer on a flash drive. It will boot into the Mac OS X installer. Just insert the CD into the target Mac and hold the "c" key as you boot up the computer. If you have the Mac OS X installer CD handy, it's super easy to change the administrator account's password. Reset Mac Password with the OS X Installer
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