Disconnect all other drives from the computer so no mistakes are made.
Stick the 10.6 disk into the machine and press the power key while holding the option key down. (holding the c key also works)
You will see a selection of boot options/images, select the 10.6 disk and click the little arrow key.
The computer will boot from the disk.
You will see a language screen, then the next screen looks like the installer.
Wait a second and look up, there is a Utilities menu and under that is Disk Utility, select that.
On the left you will see your boot drive, it will have the drive makers name and size, select that
next select First Aid then you can select > Repair Disk and Repair Permissions.
If Repair Permissions fixes something, run it again.
If Repair Permissions doesn't fix and returns a error, you need to backup your users files off the machine to a external drive and then either have your drive replaced or zero erased and everything reinstalled.