Your problem is that some files needed by the system during startup have become corrupted. The only solution is to reinstall OS X. If the drive isn't corrupted you may be able to do an Archive and Install which will not erase the drive:
How to Perform an Archive and Install
1. Be sure to use Disk Utility first to repair the disk before performing the Archive and Install.
Repairing the Hard Drive and Permissions
Boot from your OS X Installer disc. After the installer loads select your language and click on the Continue button. When the menu bar appears select Disk Utility from the Installer menu (Utilities menu for Tiger.) After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list. In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive. If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported, then quit DU and return to the installer.
If DU reports errors it cannot fix, then you will need Disk Warrior (4.0 for Tiger) and/or TechTool Pro (4.5.2 for Tiger) to repair the drive. If you don't have either of them or if neither of them can fix the drive, then you will need to reformat the drive and reinstall OS X.
2. Do not proceed with an Archive and Install if DU reports errors it cannot fix. In that case use
Disk Warrior and/or
TechTool Pro to repair the hard drive. If neither can repair the drive, then you will have to erase the drive and reinstall from scratch.
3. Boot from your OS X Installer disc. After the installer loads select your language and click on the Continue button. When you reach the screen to select a
destination drive click once on the destination drive then click on the Option button. Select the Archive and Install option. You have an option to preserve users and network preferences. Only select this option if you are sure you have no corrupted files in your user accounts. Otherwise leave this option unchecked. Click on the OK button and continue with the OS X Installation.
4. Upon completion of the Archive and Install you will have a Previous System Folder in the root directory. You should retain the PSF until you are sure you do not need to manually transfer any items from the PSF to your newly installed system.
5. After moving any items you want to keep from the PSF you should delete it. You can back it up if you prefer, but you must delete it from the hard drive.
6. You can now download a Combo Updater directly from
Apple's download site to update your new system to the desired version as well as install any security or other updates. You can also do this using Software Update.
If you cannot repair the drive do not try the Archive and Install and turn the computer off so you do not continue writing new data to the drive. You will need to get access to another Mac in order to connect the two and use
Target Disk Mode to try and access the drive and recover your personal data. The following outlines what you may need to do in this case. Although it speaks of recovering deleted files it also is relevant to recovering data from damaged hard drives.
Basics of File Recovery
If you simply put files in the Trash you can restore them by opening the Trash (left-click on the Trash icon) and drag the files from the Trash to your Desktop or other desired location. OS X also provides a short-cut to undo the last item moved to the Trash -press COMMAND-Z.
If you empty the Trash the files are gone. Recovery is possible but you must not allow any additional writes to the hard drive - shut it down. When you delete files you erase only the directory entries, not the files themselves. However, the space occupied by the files has been returned to the system as available for storage. Writing to the drive will then eventually overwrite the space once occupied by the deleted files in which case the files are lost permanently. Also if you save a file over an existing file of the same name, then the old file is overwritten and cannot be recovered.
If you stop using the drive it's possible to recover deleted files that have not been overwritten with recovery software such as
Data Rescue II,
File Salvage or
TechTool Pro. Each of the preceding come on bootable CDs to enable usage without risk of writing more data to the hard drive.
The longer the hard drive remains in use and data are written to it, the greater the risk your deleted files will be overwritten.
Also visit
The XLab FAQs and read the FAQ on Data Recovery.