I have a 20-inch iMac and it keeps telling me I need to restart. What I have tried is holding down C while I try to boot the install cd and also when I try to go to single user mode I get the message to the ilk "System OS not set" so that must be the problem. Also, it doesn't let me type anything in single user mode. And that may be because I have a wireless keyboard. Please tell me the steps to trouble shoot so I don't have to keep coming back to here and set my backup computer over and over again. Thanks, J3R.