You will need to restart your computer in something we call «single user mode» wich give you a root access to your computer (command line only). You can to that by holding CMD+S keys while rebooting. The screen should turn black and geek stuff appear on the screen. When it's finish loading, enter these command :
/sbin/mount -uw /
rm /var/db/.applesetupdone
reboot
than you'll get the wizard that you've seen when you boot the first time after Mac OS X installation. Follow all the steps including creation of an Administrator account.
basically this crack remove the file that tells Mac OS X you've already done the installation and let you create a new administrator account.