Buy a disc cleaner.
EDIT: Or if you're really ambitious, you can use a dremel tool and buff it yourself. But try it on a test disc first, if you leave it in one spot for too long you'll melt the plastic, and then you're really ****ed. Honestly, though, I've gotten games to work by doing this.
I made a flow-chart for those of us who struggle with English as a first language:
http://img202.imageshack.us/img202/2711/dumbass.png
------------------------------------------------------------
2v2 with ChainsOfAnarchy: 21-1