I fresh installed Ubuntu 12.04 then recently installed xubuntu-desktop on top of it as I don't like Unity's slowness.
Still when logging in it shows Ubuntu style login screen. Now I want to bring Xubuntu style login screen.
How can I do it?
If you don't want any of the Ubuntu applications and are good with just the Xubuntu defaults, you can use the command on this site: http://www.psychocats.net/ubuntu/purexubuntu
I got the clue from the post pointed out by @Stromson.
Just use this command:
sudo /usr/lib/lightdm/lightdm-set-defaults -g lightdm-gtk-greeter
I got Xubuntu style login screen.
One option is to re-install using an xubuntu disc instead of ubuntu as the base. Depending on how long ago you've done the install, and how many files/setting/etc you need to save, this might be the simplest way.