URGENT: OS X Gone.

[BillGM]

Today I decided to remove my Windows XP partition from my MacBook Pro and double it's size. Unfortunately, while running my Windows XP install disk, I deleted my OS X partition. I now have a barely functioning Windows XP taking up 100 GB of space and the rest of my 250 GB drive is raw space, and I can't figure out how to reinstall OS X. I have a back of my OS X partition on an external drive, but I don't see how I can use it to restore my lost OS.