Finding the correct build to re-install

you can order either new Replacement DVD discs for your system or the SL White DVD from Apple but for either you have to call Apple. None are available for Download.

There is no download for Snow Leopard. If your disk won't work, you'll have to call AppleCare & order replacements. There are two disks, an Install Disk & an Applications Disk. They will charge you a small fee to send out replacements.

You can't download Snow Leopard but you can get a machine-specific replacement disc from Apple by calling 1-800-MYAPPLE. I forget what the cost is but it's not much. But it sounds as if you need to get your optical drive replaced first - if it's the culprit in scratching your CD/DVDs.

Clinton

Ok you have the original DVDs and a Snow Leopard 10.6.3 DVD. So it seems your internal (Not So) Superdrive is the problem. Then buy a External DVD drive that connects to a USB port to do the install. Any brand will work, It does not have to an Apple branded drive.

One of these Tray load external DVD drives may even read the original system disc even if it does have a slight scratch on it.

IMHO the included Superdrive that Apple uses is one of the worst drives I have ever used. In fact on my late 2011 15" MBP I have stopped using it for any critical work that requires the use of reading or writing CD or DVD discs and only use it sometimes to rip music CDs.