Which "kid" had physical access to your computer, which this requires? You should be able to go back in just the way he did, but using step one, since you should know the name of your user. If that won't work, then step 2. It's not really a "hack;" It's more a well known vulnerability, if you allow physical access to the computer.
The computer wasn't "reregistered."
This is obviously someone you know, so I would confront him or her with it.
EDIT: Yes, of course, use Topher's method with the install DVD.
Don't let the Firmware Password give you a false sense of security. Again, with physical access to the computer, the Firmware Pasword can easily be defeated. It sounds like you really need to take control of the situation, more than anything else. You also need to find out what else might have been changed during the time you lost control.
Message was edited by: WZZZ
You can use the OS X installation DVD to run the password reset utility and change the password for the new Admin account that was created. This in essence pulls the same "hack" that was used to change the account in the first place (see here for ways of doing this: http://reviews.cnet.com/8301-13727_7-20044353-263.html).
When done, you can prevent booting to Single User mode or to another boot volume by enabling a firmware password on the system (see the "how to enable a firmware password" section in this article: http://support.apple.com/kb/HT1352).