On apt-get on a 11.10 machine i'm getting:
W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/oneiric-security/main/source/Sources 404 Not Found [IP: 91.189.91.14 80]
W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/oneiric-security/universe/source/Sources 404 Not Found [IP: 91.189.91.14 80]
W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/oneiric-security/multiverse/source/Sources 404 Not Found [IP: 91.189.91.14 80]
W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/oneiric-security/main/binary-amd64/Packages 404 Not Found [IP: 91.189.91.14 80]
W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/oneiric-security/universe/binary-amd64/Packages 404 Not Found [IP: 91.189.91.14 80]
W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/oneiric-security/multiverse/binary-amd64/Packages 404 Not Found [IP: 91.189.91.14 80]
W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/oneiric-security/main/binary-i386/Packages 404 Not Found [IP: 91.189.91.14 80]
W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/oneiric-security/universe/binary-i386/Packages 404 Not Found [IP: 91.189.91.14 80]
W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/oneiric-security/multiverse/binary-i386/Packages 404 Not Found [IP: 91.189.91.14 80]
Indeed, security.ubuntu.com has packages for other releases, but not 11.10. What happened to them?
It has reached EOL status a year ago and therefore removed from the main repositories. Update the system as soon as possible to something supported.
Other than that, archives for old releases can be found here.