Sounds like an important file got corrupted when the power was pulled. You could try booting up from the restore disc (put the disc in, reboot and hold down the 'c' key) then run Disk Utility to repair permissions and check the drive. Also reset the admin/user passwords while you're at it.
If that fails, it really depends on how much time you want to spend faffing around. You may be able to get it back to normal by re-installing certain files, copying files from other macs, etc but this can be quite tedious.
What I would do - Again, using the the restore disc, you can perform an Archive & Install which should keep all your files and settings but re-install the main system files. That MIGHT do the trick.