You need to use the installation discs that originally came with your machine. If you've lost them, you can call Apple and, with the srial number of your machine handy, they can send you a set of discs that came with your MBP. No idea what the cost is - $25 or so? - but uou'll be able to use disc one to erase your hard drive for a fresh install of the OS that came with your machine and then upgrade to Snow Leopard using your SL disc (which I'm assuming you have? If not, that can be ordered via the Apple Online Store).