Can't boot Mac, only shows a Folder with a Question Mark?
I just went through this my self!

Your Mac can't find anything to boot from. Or more specifically, it can't find the system folder on your primary boot device.

Try booting from your OS X DVD and running Utilities -> Disk Utility to check your boot drive for errors.

To boot from your DVD, insert in to Mac, turn off Mac, and hold C while you turn it on. You have to hold it until it chimes IIRC. That'll tell it to boot from the DVD.