How do I change the default log-in to Xubuntu on Ubuntu 12.04?

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http://askubuntu.com – I thought I would set up my girlfriend's tired old netbook running a less broken OS than Linspire. Nice idea. I have a similar machine ( Acer Aspire One ) running Ubuntu and that worked fine. However, when I installed it on hers, it decided to use Unity3D and nothing else. She has it set up to log her in automatically and it seems that no matter how much I set up /etc/lightdm/lightdm.conf to have user-session=xubuntu it does nothing. It shows the Xubuntu loading screen, then pops her into a painfully slow Unity3D desktop that her system cannot support at all. There doesn't seem to be anyth (HowTos)