How to reinstall Snow Leopard on my MBP?
Just put in the Install DVD.
You can do that with the computer running and run the installer from the disc.
You can also boot directly to the disc by turning off the computer, putting it in, holding down the c key, and turning the computer on.

Follow the directions, and you'll have a clean install. Your personal files and preferences will be saved, but system files will be replaced.