Comment 2 for bug 579023

Seems the unresolved Emerald upgrade will keep on nagging with error messages in Synaptic or apt-get.

I did a Google search for the Emerald issue and discovered that Emerald is not supported on 10.04. (see http://reformedmusings.wordpress.com/2010/05/08/emerald-no-longer-supported-by-compiz-in-ubuntu-lucid-10-04-lts/)

I solved (well partly, as the solution is to replace Emerald with GTK ) the issue by starting GTK as windows decorator and then dis some manual tricks to have Emerald removed:
gtk-window-decorator --replace &

I then tried a shell-based removal :
sudo apt-get remove emerald

Got some errors :
Removing emerald ...
update-alternatives: error: while reading /var/lib/dpkg/alternatives/x-window-decorator: Is a directory
dpkg: error processing emerald (--remove):
 subprocess installed pre-removal script returned error exit status 2
update-alternatives: error: while reading /var/lib/dpkg/alternatives/x-window-decorator: Is a directory
dpkg: error while cleaning up:
 subprocess installed post-installation script returned error exit status 2

I then removed the files in the /var/lib/dpkg/alternatives/x-window-decorator directory:
sudo rm -f /var/lib/dpkg/alternatives/x-window-decorator/entry.desktop
sudo rm -f /var/lib/dpkg/alternatives/x-window-decorator/flag.png

Removed the directory and created an empty file:
sudo rmdir /var/lib/dpkg/alternatives/x-window-decorator
sudo touch /var/lib/dpkg/alternatives/x-window-decorator

Rerunning apt-get:
sudo apt-get remove emerald

Removed Emerald successfully.