I have a 24" iMac G5 intel. I tried to upgrade to Snow Leopard from Leopard and firstly I got an error message saying that I needed to clean the install disk. I did that and in fact changed the disk for another at the store. Since then I have done five clean installs and even managed to get the computer running until it had to close from a software update. Restarting I get to the grey screen with the Apple logo and the spinning wheel. After 5 minutes it shuts down and restarts, then the event is repeated. I have tried clearing the PRAM etc, but now I can't even boot to the Install disk. I'd be grateful for any guidance please.