That trick does one thing, it makes Mac OS X think its not completely installed, which makes it ask you to create a new user account at boot. You will then have access to this account and be able to modify all other accounts.
- Boot your PC and hold down Apple and C immediately after the boot sound. When you get into the console, you will have to type commands with your keyboard in full conformity with the case.
- Type mount -uw / and press Enter.
- Type rm /var/db/.AppleSetupDone and press Enter.
- Type shutdown -h now and press Enter.
- The computer shutdowns, you can then reboot it, answer the questions, create your new user account and then access other Accounts in the System preferences to change their passwords.