I am having this same error.  I've reloaded a few times and used a wired connection over a wireless one but nothing seems to work.  

Here's my /etc/apt/sources.list:

## See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
## newer versions of the distribution.

## Add comments (##) in front of any line to remove it from being checked.
## Use the following sources.list at your own risk.

## Uncomment deb-src if you wish to download the source packages

## If you have a install CD you can add it to the reposity using 'apt-cdrom add'
## which will add a line similar to the following:
#deb cdrom:[Ubuntu 7.04 _Feisty Fawn_ - Beta i386 (20070322.1)]/ feisty main restricted
deb cdrom:[Ubuntu 7.04 _Feisty Fawn_ - Release i386 (20070415)]/ feisty main restricted
deb http://us.archive.ubuntu.com/ubuntu/ feisty main restricted
#deb-src http://us.archive.ubuntu.com/ubuntu/ feisty main restricted

## Major bug fix updates produced after the final release of the
## distribution.
deb http://us.archive.ubuntu.com/ubuntu/ feisty-updates main restricted
#deb-src http://us.archive.ubuntu.com/ubuntu/ feisty-updates main restricted

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team, and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also, please note that software in
## universe WILL NOT receive any review or updates from the Ubuntu security
## team.
deb http://us.archive.ubuntu.com/ubuntu/ feisty universe
#deb-src http://us.archive.ubuntu.com/ubuntu/ feisty universe

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team, and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also, please note that software in
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb http://us.archive.ubuntu.com/ubuntu/ feisty multiverse
#deb-src http://us.archive.ubuntu.com/ubuntu/ feisty multiverse

## Uncomment the following two lines to add software from the 'backports'
## repository.
## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
deb http://us.archive.ubuntu.com/ubuntu/ feisty-backports main restricted universe multiverse
#deb-src http://us.archive.ubuntu.com/ubuntu/ feisty-backports main restricted universe multiverse

deb http://security.ubuntu.com/ubuntu feisty-security main restricted
#deb-src http://security.ubuntu.com/ubuntu feisty-security main restricted
deb http://security.ubuntu.com/ubuntu feisty-security universe
#deb-src http://security.ubuntu.com/ubuntu feisty-security universe
deb http://security.ubuntu.com/ubuntu feisty-security multiverse
#deb-src http://security.ubuntu.com/ubuntu feisty-security multiverse

## PLF REPOSITORY (Unsupported.  May contain illegal packages.  Use at own risk.)
## Medibuntu - Ubuntu 7.04 "feisty fawn"
## Please report any bug on https://launchpad.net/products/medibuntu/+bugs
deb http://packages.medibuntu.org/ feisty free non-free
#deb-src http://medibuntu.sos-sts.com/repo/ feisty free non-free

## CANONICAL COMMERCIAL REPOSITORY (Hosted on Canonical servers, not Ubuntu
## servers. RealPlayer10, Opera, DesktopSecure and more to come.)
deb http://archive.canonical.com/ubuntu feisty-commercial main

## enlightenment e17 beta, use at your own risk
## E17 is in Beta and may break or break your system
#deb http://edevelop.org/pkg-e/ubuntu feisty e17
#deb http://e17.dunnewind.net/ubuntu feisty e17
#deb-src http://edevelop.org/pkg-e/ubuntu feisty e17