I try to restart my computer and hold C on start up to start up on Snow Leopard Disc, and it begins to show the loading circle, but then comes down telling me to restart my computer by holding down the Power Button. When I turn the computer back on and it loads to the OS, it tells me it shut down based on a problem or something like that and to "ignore, or report" What is going on? It's worked before. It should start up to show options to install snow leopard and wipe the disc, etc etc. Fresh install or upgrade. I tried startup with holding option and selecting the disc as well, no go. I cleaned the disc and it's not scratched or anything. What is going on?