software center - How do I unlock /var/lib/dpkg/lock?

I attempted to install dropbox on my Ubuntu system, but now I am unable to install or uninstall any software. I get this error:

E: Unable to lock the administration directory /var/lib/dpkg/lock?

I have already tried killing dpkg and restarting it sudo kill -TERM PID but it immediately tries to finish the installation again and gets stuck.

How do I resolve this?

sudo rm /var/lib/dpkg/lock should do the trick. You can use ps afx|grep dpkg to check if there is still a process runnning at the same time. sudo killall dpkg will stop all running dpkg processes.

If you need to cancel the dropbox installation, try
sudo dpkg --force-remove-reinstreq --remove nautilus-dropbox