I use Ubuntu 13.04 32bit in a dell studio 1537. Its been a couple of days, I see a error/warning sign (a red triangle with exclamation sign in it) on the top bar. Clicking on it the message says, "The update information is outdated. This may be caused by network problems or by a repository that is no longer available. Please update manually by clicking on this icon and then selecting 'Check for updates' and check if some of the listed repositories fail." There is no network problem since internet works fine. When I check for updates, shows your computer is up to date
Here is the output of sudo apt-get update