OS X Snow Leopard - Can't boot from HD, Target Mode or CD
(Just to provide an actual answer that you can accept).

It looks like so far you've not actually tried accessing the Boot selection menu. Rather, you've mistakenly entered target disk mode.

Press and hold the alt key immediately after powering on the system (before you hear the chime). You should get a selection of the available boot volumes (if any). If you have an external drive or install DVD, this should appear. Use the arrow keys to navigate to your choice and hit enter to boot from that volume.

Use your alternative boot to backup data and repair/reinstall your main boot disk.