Before using a third-party disk utility, consult the precautions cited in the "Using disk utilities to correct hard drive problems" section of this FAQ.
Bad sectors are the result of problems with the magnetic media used to save data on a hard disk.
No disk utility is a substitute for a good, current backup.
The potential for data loss, despite the excellent reliability of modern hard drives, is why it is critical to implement a comprehensive Backup and Recovery solution and use it regularly before you need it.
Detailed instructions on reinstalling Mac OS X can also be found in our book Troubleshooting Mac OS X.
Otherwise: Create a new backup of the affected Mac's startup disk using your comprehensive Backup and Recovery solution.
Hard drive failures, bad sectors, and directory corruption are serious problems as these can result in data loss.
If you value your data, you have implemented a comprehensive Backup and Recovery solution and use it regularly, preferably backing up everything on a daily basis.