Around 55GB
Hold down shift while your computer starts up. This forces the system to check your startup volume.
Also try holding down D with the your first restore disc in the drive when your computer starts up. This will prompt the computer to run the Apple Hardware Test. This will help determine if your hard drive is at fault.
You can also try booting from your Macs hard drive with another Mac via target disk mode.
Also try what tallest skil said too. Boot from the restore disc and verify the permissions.
- Holding down SHIFT boots to Safe Mode, I don't think it "checks" the startup volume.