MAC install problem

I'm visiting a girlfriend of mine and she somehow manage to delete the type of files that aren't designed to be deleted. This is on G4 laptop. The thing is that everytime you click on something to execute it it tells you that the file isn't there. even if the file is on a cdrom. She is now running like os 9.some BS and she needs to get back onto os 10. Can anyone tell me how to reboot. Format, and fresh install the OS 10 off disk? I've had the disk in the drive and it just keeps on booting strait into OS 9.BS and that's it. So if any of you can help out please gimme a ring.
Duck

I believe there is a key you have to hold down to bootfrom the cd. I don't remember which off the top of my head (maybe it's c). Try checking around apple's website.

it's c.