Are you doing a clean install (erase & install), archive & install (bring over old users to clean install, creating a "Previous Systems" folder), or just a plain old upgrade install (avoid!)? Try archive & install, and if that doesn't work, you may need to wipe the drive and just start over.
You can also just try doing a PRAM reset (command-option-P-R until second boot beep at startup) and see if that helps. Or if you can ever get to a desktop, try repairing permissions.
Steve