Repair Disk

Repair Disk (Insert Installer disk and Restart, holding down the "C" key. When booted from the installer disk go to the Installer Menu and select Disk Utility. Select your HDD (manufacturer name) in the left panel and First Aid in the Main panel. Click Repair Disk on the bottom right. If DU reports errors run Repair again, and again until no errors are reported.
If DU reports errors it cannot repair you will need to use a third party utiity like Tech Tool Pro or Disk Warrior)