Keyboard Startup Options

You can hold down the C key at startup to start up from a bootable CD or DVD already in the drive. Holding down Command-Option-Shift-Delete forces the Mac to bypass the typical default startup drive (your internal drive) in favor of an alternative bootable device, such as an external FireWire drive. This may succeed in cases where holding down the Option key does not access the drive you wish to use.

Excerpted from Mac OS X Help Line, Tiger Edition