Thanks for all of your helpful messages! Feels like a friendly world when my computer is acting decidedly unfriendly.!
I posted the whole list of what the Apple folks asked me to do under Macbook - app update failure.
last thing with those guys they asked me to do power up while holding the option key with the OS disc in the drive, selecting that. It didn't work immediately so they said, take it to one of the Apple stores.
Tried it again this evening and It did work so I guess I will call them again in the morning and see what it takes to re-install Snow Leopard. Hopefully my drive has not died.
Thanks again!