Disk Utility and write protected disk

I'm trying to repair my start up disk in Disk Utility, but the "repair disk" command is dimmed. The Disk Utility Help page says this is because the disk is write protected. So how do I un-write protect the disk so I can repair it using Disk Utility? Running Tiger on a G4 iMac. :ninja:

Boot to an OS X CD to run a repair.

You can't repair a booted volume.