restart your computer, hold down Command-S.
( Basically, you hold down the command-s key when powering on. The command key has a little apple symbol on the lower left. It is between the alt/option key and the space bar. On a PC keyboard, it will be the windows key, I think. )
When you get the prompt, type in the following:
/sbin/fsck -y [Enter]
/sbin/mount -uaw [Enter]
rm /var/db/.applesetupdone [Enter]
reboot [Enter]