External moniter issues- trying to install Snow Leopard...

Hey y'all.

The Details
-I have a 17 in. Macbook Pro, late 2007 model
-The screen is cracked to the point where I cannot see anything.
-It's hooked up to an external monitor

I've just purchased Snow Leopard, because frankly I'm sick of being stuck at V. 10.4.11, and it's got a lot of features that I'd find useful.

I have tried to install it, but when I restart, all I see is a purple galactic background of some kind and nothing else. This is the background it displays to install Snow Leopard, and the display is hidden somewhere offscreen, perhaps even fixed to it (which would make it impossible to drag). So, I gave up until now, a few days later, and a mac-savvy friend of mine is showing me how to access mirrored displays and how to turn it off, and I essentially re-create the conditions I experienced when I restarted to install Snow Leopard. It's basically turning off mirroring. I've tried hitting F7 and nothing happens.

So my problem is, can I turn it back on with a few other keyboard prompts? Is there a way to force the mirrored view to come back on at all? How do I do that?

Thanks for your help.