Ubuntu Software Center Error

Please try this:

Open a Terminal from the menu Applications->Accessories->Terminal

Tip: right click with mouse on the terminal title caption and select the item "Always on Top" doing this you will force the terminal window to stay on top of the other windows and you will find very easy to copy single row from this web page into the terminal...
Something more about using the terminal https://help.ubuntu.com/community/UsingTheTerminal

Then type or better copy and paste a row a time then press enter:
(Tip: select the single row to copy then right click into the terminal and to quick paste click with middle button of mouse )

sudo cp -arf /var/lib/dpkg /var/lib/dpkg.backup
sudo cp /var/lib/dpkg/status-old /var/lib/dpkg/status
sudo cp /var/lib/dpkg/available-old /var/lib/dpkg/available
sudo rm -rf /var/lib/dpkg/updates/*
sudo rm -rf /var/lib/apt/lists
sudo mkdir /var/lib/apt/lists
sudo mkdir /var/lib/apt/lists/partial
sudo aptitude clean
sudo aptitude autoclean
sudo apt-get update
sudo dpkg --clear-avail
sudo dpkg --configure -a
sudo aptitude install -f
sudo aptitude update
sudo aptitude dist-upgrade
sudo apt-get --reinstall install gimp

give your user password when requested, you don't see nothing when you type it, then press enter.

Hope this helps