How to restore Ubuntu login screen after lubuntu install?

I have Ubuntu 13.10 installed. I then installed lubuntu 13.10 (sudo apt-get install lubuntu-desktop) too. As a result the login screen has changed and I can no longer get an on screen keyboard.

How can I restore the login screen to the original Ubuntu default?

Since the OP is a bit lazy(!), I'm going to post the answer myself. Most of the times, thesudo dpkg-reconfigure lightdm solution doesn't work. You'll need to change the settings manually either by using lightdm-set-defaults or editing /etc/lightdm/lightdm.conf.

• Open up lightdm.conf:

  gksudo gedit /etc/lightdm/lightdm.conf
  

• You must have the following settings under [SeatDefaults]:

  user-session=ubuntu 
  greeter-session=unity-greeter
  

• Save and reboot.

My login screen was changed after I installed KDE. Anyway, one possible solution (what worked for me because in my research I found others but they didn't work) was to uninstall and reinstall lightdm:

sudo apt-get purge lightdm
sudo apt-get install lightdm

Of course, I had to specify what display manager I want (gdm, kdm or lightdm - after I broke my lightdm I used gdm as default). For this you have to run

sudo dpkg-reconfigure gdm