Try cleaning the disc. If that doesn't work, maybe the disc is scratched badly. If that's the case, then you can get the disc re-surfaced. Try asking at game stores and specialty CD stores. It's a pretty cheap process that replaces the plastic layer on the front of the disc. Takes away all scratched, leaving a disc that's as good as new.

You don't need to waste money on a replacement game unless the scratches are really bad and go all the way through to the label side of the disc.