Go to the Apple Store and ask if they can reinstall your OS, because you're disks got scratched. It's worth a shot, if it doesn't work at least you're already there to buy OS X again...
There's other not legal options that, if you really own the OS, some might argue the morally of, but that doesn't change the fact that it's against the law and the wrong thing to do(anyone who pirates anything is a cheap d-bag, reguardless of how they try and rationalize it to themselves and others).