Going back to a previous kernel of Ubuntu

gistfile1.txt

Fire up the terminal and run this command:
dpkg -l | awk '/linux-[^ ]+-[0-9]/ {print $2}'

It should print all of the available kernel versions.

Check which version of kernel you are currently running by typing in: uname -a

Once you know which new version of the kernel you want to remove type:

sudo apt-get purge linux-headers-3.8......
sudo apt-get purge linux-image-3.8......

Note: it is possible to type 'apt-get -s purge ......' to see what will be done without actually running the command.