I compared yours to mine, and there are
differences. Some of them seem unimportant. I'll copy the different
lines from your sources list and would appreciate it if you'd advise on
whether I should change my sources list to match yours. I think seeing
the differences will be more effective than me explaining. I've only
copied those that are missing in mine or different(and these are the
majority)

These are the url's I don't have
deb http://ppa.launchpad.net/do-core/ubuntu gutsy main
deb http://us.archive.ubuntu.com/ubuntu/ gutsy-updates universe
deb-src http://us.archive.ubuntu.com/ubuntu/ gutsy-updates universe
deb http://us.archive.ubuntu.com/ubuntu/ gutsy-updates multiverse
deb-src http://us.archive.ubuntu.com/ubuntu/ gutsy-updates multiverse




In these the word "US" doesn't exist - I presume that is because I'm not
in the States? Mine starts with the word "ARCHIVE". The rest of the url
is the same.
deb http://us.archive.ubuntu.com/ubuntu/ gutsy universe
deb-src http://us.archive.ubuntu.com/ubuntu/ gutsy universe
deb http://us.archive.ubuntu.com/ubuntu/ gutsy multiverse
deb-src http://us.archive.ubuntu.com/ubuntu/ gutsy multiverse
deb http://us.archive.ubuntu.com/ubuntu/ gutsy-backports main restricted
universe multiverse
deb-src http://us.archive.ubuntu.com/ubuntu/ gutsy-backports main
restricted universe multiverse


These aren't offered to me but I see you don't use them as they are with
a hash
# deb http://archive.canonical.com/ubuntu gutsy partner
# deb-src http://archive.canonical.com/ubuntu gutsy partner


I have all these url's BUT the word "SECURITY" is replaced in mine by
the word "ARCHIVE". The rest of the url is the same
deb http://security.ubuntu.com/ubuntu gutsy-security main restricted
deb-src http://security.ubuntu.com/ubuntu gutsy-security main restricted
deb http://security.ubuntu.com/ubuntu gutsy-security universe
deb-src http://security.ubuntu.com/ubuntu gutsy-security universe
deb http://security.ubuntu.com/ubuntu gutsy-security multiverse
deb-src http://security.ubuntu.com/ubuntu gutsy-security multiverse

Comments??