-Rinse it off with water and pat dry with a soft towel.
-Take toothpaste (paste works best, not gel) and use a soft cloth (like a goggle case) and take a small amount of toothpaste and rub the scratch in outward radial motions. Do this until it looks like the scratches are pretty much gone.
-Rinse the disc off until clean and pat dry with a towel again
-Try it in the xbox and see if it works. If not, repeat.
I have done this with a few discs and it always works. I actually recently did it with my MW2 disc that got scratched and wouldn't play.