Can't boot from DVD or external HD... anyone experienced this?

As the title says, I cannot boot my mid 2010 MBP 15" i7 from the install disc, a snow leopard DVD, or even from an external hard drive imaged with the snow leopard disc.

I originally discovered the problem when I ran a verify disk and was given errors. It told me it couldn't be fixed unless I started from the install disc. After a ton of failed attempts (after pressing the C key.. you just see the grey screen with the Apple, but it never goes anywhere) I started my computer, connected by firewire to my girlfriends in target disk mode and fixed the drive that way. I thought this might correct the problem but still no luck.

I have no need to boot from the disc immediately now, but should I ever want to re-install in the future, I'm going to need to be able to do this. Anyone have any idea what the problem might be?

Thanks in advance!

 

I did do that

The problem is, that even after doing that, I still cannot boot from DVD, or external HD. I'm trying to figure out what might be causing that problem.

Thanks