I have no experience of Macs. I have just received one for repair. Screen smashed, unreadable. Machine boots, but HDD sounds like a bag of nails for about 10 secs then calms down. Maybe kaput, maybe not. I can replace the screen no bother, but how do I replace the OS? 2 possibilities, if the HDD is readable, copy it using a PC if that's possible, and re-install to a new drive. Otherwise download OS X 10.4.6? From where? Do you need Product keys like Windows? Any help appreciated. Meanwhile have requested owner to search for Install disks, but as it's approaching 8 years old, may not happen...