Well, I followed the advice of some of you nice folks and bought a used replacement internal hard drive for my mid 2007 iMac. I was able to put the drive in. But now I am having trouble with the OS. The hard drive came out of a Windows machine and so when I turn it on, I get a windows screen. When I turn i on with option key pressed, I only get the option for the windows hard drive. I was able previously to reinstall the OS over the internet directly from Apple. But I get the feeling that that is not an option with this old mac now that it doesn't have OSX installed on its hard drive. I have a macmini with Mavericks on it. I have 16GB thumb drives. I have firewire cables. ( I read something about Target mode.) I had Mavericks on the iMac also. I don't have any CDs. What am I supposed to do? Thanks!