This article is about what to do when :

• I got this Mac and the previous owner didn't or add me as new admin user or gave his/her/its/their password. So I can't open the [LOCK] in the configuration panels.
• I forgot my password for this Mac
• I just want to have a sparkling new clear desktop and adding a new admin account and want to keep the old user account. (which can be deleted later..) And Bonus: I get to see that fancy welcoming animation.

What do I need to do:

1. Start/Restart the mac
2. Press the command key⌘  and [S] together , keep it pressed untill you are in the terminal on the screen.
3. Type  /sbin/mount -uaw  and [ENTER]
4. Type rm /var/db/.applesetupdone  and [ENTER]
5. Type reboot and [ENTER]

This will trick the Mac OS X thinking it was just installed as new , like you just unboxed your Mac ! 

The old account stays unharmed, nothing will be deleted.

Your system will reboot, and you will be welcomed with the welcoming animation and a big X ( 10.5 ) and then asking you a few questions.

Enter your location, information, follow all steps. You can skip the Apple ID and registration step, simply leave the fields blank.

If everything is done, you will arrive on your new desktop.

In the configuration / user panels you can change the password of the old user account and use it again or simply remove the whole old account. (including documents)

You also can takeover ownership of the folders in the old user account and copying it over to the new account.

Great job !

⌘  , what is this and where can I find it?

It's the command key, also called 'apple' key. You see an key with an apple on it? Yes? Ah, that's the command key!

Still not sure? Ok, here's an GRAPHICAL one.