Boot Mac OS X Into Single User Mode by holding down Command + S on system boot.

Once the system has booted into its terminal looking single user mode simply type the following:

NOTE: Where it says USERNAME below simply replace that with your Mac OS X main admin login username.

$ mount -uw /
$ rm -R /Library/Preferences/$ rm -R /Users/USERNAME/
$ /bin/launchctl load /System/Library/LaunchDaemons/com.apple.DirectoryServices.plist &$ dscl . -delete /Users/USERNAME$ rm /var/db/.AppleSetupDone
$ exit