Mac OS X 10.6: Repairing a disk
You may need to repair your disk if you experience a problem such as an application quitting unexpectedly, a file becoming corrupted, an external device not operating properly, or your computer failing to start up. You can repair your disk with Disk Utility.
To repair your startup disk:
Open Disk Utility, in the Utilities folder in the Applications folder.
Select your disk in the list at the left, and then click First Aid.
For more information on repairing your disk, click the Help button (looks like a question mark).