Put your OS CD into the drive, and start computer
Hold down the 'C' key until you see the Apple Logo on screen. You will be able to hear the CD drive spin up and start to load the operating system.
You need to hold the 'C' key down to this point as it will boot from the Hard Drive otherwise.
It will take a looong time to boot from the CD, be patient.
When it comes up with the 'Select language' accept that and it will proceed to the installer window.
Go to the Menu at the top of the Screen and select 'Utilities' - select 'Disk Repair'.
Select your base hard drive on the LHS-usually the one at the top.
When you highlight this then choose 'Repair Disk' at the botto RHS.
Again be patient.
That should be it.