How to fix this error: "E: Some index files failed to download. They have been ignored, or old ones used instead"? [duplicate]

When I try to run the following command in Ubuntu 13.04:

sudo apt-get update

I got following error:

W: Failed to fetch com/xampp/ubuntu/dists/raring/main/binary-i386/Packages 404 Not Found
E: Some index files failed to download. They have been ignored, or old ones used instead