You need to use Disk Utility to Repair Disk. There are two ways to do that:
1) Get a replacement Restore Disk from Apple. They will send you a replacement for about $20 or you can get one at your local Apple Store for a little less (maybe free if you bring a blank DVD and are really, really nice - they burn them right there in the store from an archived disk image and while they are SUPPOSED to charge you, I've brought a blank DVD and they did it free). Once you have the disk, boot from it and open Disk Utility.
2) If you have another Mac you can start your current one in Target Disk Mode (by holding down the T key while you start it up). Then connect it to another Mac via FiewWire - it will mount on the other Mac's desktop as a external drive. Then open Disk Ute on the 2nd Mac and select the 1st Mac's hard drive as the disk to repair.
Good luck!
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