Restart the Mac holding down the Command+S keys, to enter
Single User Mode and the Terminal interface


In Terminal, type:

mount -uw /


and hit return.

Then enter:

rm
/var/db/.applesetupdone


And finally:
reboot

On reboot, you should see the "Welcome Wizard" screen that brand new Macs start up with.
Follow the wizard and create a new user account with a temporary name.


Boot Mac OS X with this newly created account, which will have administrative access.
Navigate to "System Preferences"(under Apple menu) and click on "Accounts".


Click on the Lock icon, and enter the new user's credentials.


Select the old user account, and click on the
"Reset Password" button


Enter a new password for the old account. Write it down somewhere.


Close System Preferences, and reboot.


You can now login to the old account using the
new password.
Delete the temporary
account, and you're in business. Restart the Mac holding down the Command+S keys, to enter Single User Mode and the Terminal interface
In Terminal, type: mount -uw /
and hit return.

Then enter:
rm /var/db/.applesetupdone


And finally:
reboot

On reboot, you should see the "Welcome Wizard" screen that brand new Macs start up with.
Follow the wizard and create a new user account with a temporary name.
Boot Mac OS X with this newly created account, which will have administrative access.
Navigate to "System Preferences"(under Apple menu) and click on "Accounts". Click on the Lock icon, and enter the new user's credentials.
Select the old user account, and click on the "Reset Password" button
Enter a new password for the old account. Write it down somewhere.
Close System Preferences, and reboot.
You can now login to the old account using the new password.
Delete the temporary account, and you're in business.