Why does an error appear every time I try to open the Ubuntu Software Center? [duplicate]

There is a glitch on the Ubuntu Software Center and whenever I open it an error appears and it keeps loading and never opens. Why does this happen? I have installed Ubuntu 13.04 on a disk and partitioned it. Please help me and ask for excess information if you need it. If you know of any duplicates please show me them!!

I get the following error in a terminal

SystemError: E:Type '<!DOCTYPE' is not known on line 1 in source list /etc/apt/sources.list.d/medibuntu.list

The /etc/apt/sources.list.d/medibuntu.list file contains a HTML document.

The Medibuntu repository, which is referred to in your SystemError output, has now been shut down, the packages were either obsolete, unnecessary or moved to the official Ubuntu archive. Now that you can no longer pull packages down from the Medibuntu archive, you no longer need the medibuntu.list file in the /etc/apt/sources.list.d/ directory.

1. Open the terminal and open the /etc/apt/sources.list file for editing in nano editor:

   sudo nano '/etc/apt/sources.list'
   

   Delete any lines in sources.list that have medibuntu in them. Press the keyboard combination Ctrl+O and after that press Enter to save the sources.list file. Press the keyboard combination Ctrl+X to exit nano.

2. Remove the medibuntu.list file from its location in the /etc/apt/sources.list.d/ directory. This command will create a backup directory and save a backup of the medibuntu.list file called medibuntu.list.bak in it:

   sudo mkdir backup && sudo mv '/etc/apt/sources.list.d/medibuntu.list' '/etc/apt/sources.list.d/backup/medibuntu.list.bak'
   

3. Tell your system to update the latest list of software from each archive it knows about:

   sudo apt-get update