Deleted /etc/, can't boot from DVD
See Startup key combinations for Intel-based Macs (HT1533) for a list of available key combinations during startup. In your case you should use either of


C: Start up from a bootable CD, DVD, or USB thumb drive (such as OS X install media)
Option ⌥: Start up in Startup Manager, where you can select an OS X volume to start from. In your case the volume to select is the OS X Snow Leopard DVD