Try using Disk Utility to repair your startup disk (hard drive) as shown in Dr.Smokes Resolving Disk, Permission, and Cache Corruption. His link will guide you thru what Allan was talking about.
As Allan said, you'll most likely need DiskWarrior to fix that problem. If you buy it, get the CD, not the download, so you can bootup on the DW CD.
If you have a good backup, you can do the following to fix it;
This is the best way to Erase and Install. First backup all your important stuff, if you can, as the following will erase everything on your drive; it will be unrecoverable. Put your install disk in your Mac and Restart while holding down the C key. In Tiger, When you get to the install screen, don't click install and go to the title bar at the top of the screen and click on Utilities (in earlier OS's click on the installer menu).
Click on Disk Utility and choose the hard drive you want your OS on. Then click on the Erase tab. In Tiger, click on the Security Options button near the bottom (it's similiar for Panther and Jaguar).
Once in there choose Zero Out Data (write zero's in earlier versions). This will map out any bad blocks on your drive and bring it back to almost new condition (providing there's nothing wrong mechanically with it, bad bearings, defective or damaged surface, etc.).
Once this is done, go back to the install screen and begin the "Erase and Install" installation. This will put a factory fresh system on a clean hard drive.
Cheers!
DALE