You may have added a custom repository to your software sources, in which the above software was available; that repository may be offline now. Here's what you can do to check:
Originally Posted by HunkieChan
Open a terminal (Applications-Accessories-Terminal), and give the command
You will get lots of output, check it for errors manually.
sudo apt-get update
If you find a certain repository is giving errors, note it's name and then comment it out from the "/etc/apt/sources.list" file. You can edit the file with the command
which can be given either in the terminal or the Alt+F2 ("run") box.
gksudo gedit /etc/apt/sources.list