yesterday I stopped a download before it was finished.
Now when I go to ADD/REMOVE I just get an error message.

Failed to check for installed and available applications.

This is a major failure of your software management system.
Please check for broken packages with synaptic, check the file
permissions and correctness of the file '/etc/apt/sources.list'
and reload the software information with 'sudo apt-get
update' and 'sudo apt-get install-f'

I know that the problem is mainly my lack of knowledge. I would really appreciate it if someone could explain to me what is going on, and how to remedy the situation.

Thanks in advance

Can you post the contents of your /etc/apt/sources.list file? The source (sorry) of your problem appears to be with that file. Mentioning your distro and version would be helpful as well.

Remember you have to be root or use sudo when adding/updating software.
Cheers,
jdk

Ubuntu 9.04

Please tell me how do I post it.

Either as an attachment or just copying and pasting the contents in a text box will be fine.
cheers,
jdk

I typed sudo apt-get -f install then was able to complete the installation of the program. Don't ask me what is going on, I don't have a clue