Are you running Lion or an earlier version of OS X?
Lion users can boot Lion Recovery and reset it thus:
- Shut down your Mac and turn it back on while holding down the ⌘ and R keys at the same time until you see the Lion Recovery screen.
- From the menu bar at the top, select Utilities > Terminal.
- In the Terminal window type resetpassword and press Enter.
- Use the Reset Password utility to select your startup volume, choose your user account, and enter a new password.
Earlier OS versions:
Mac OS X v10.6 or later: Choose Password Reset from the Utilities menu.
Mac OS X v10.5 or v10.4: Choose Reset Password from the Utilities menu.
Mac OS X v10.3: Choose Reset Password from the Installer menu.