I'm having trouble installing a brand-new copy of Snow Leopard onto my Macbook. It loads just fine, and I'm able to start the installation. Halfway through the process, the computer restarts itself and cancels the installation. Occassionally I get a warning sign from the installation program that says it can't read the disk and that it needs to be cleaned. The disc is brand new and isn't scratched so i'm not sure why it's giving me that warning. Anything I can do to fix this or do I need to get the disc replaced?