Upgrading to Snow Leopard without an Optical Drive

"I installed it using an external HDD, because my Superdrive was malfunctioning at the time. However, you'll still need access to a disc drive so that you can rip the original Snow Leopard disc.

Rip the original disc to a disk image, and then partition your external drive. Open Disk Utility and restore one of the partitions using your disk image of Snow Leopard. Boom. Bootable partition."