Tente isto:
Abra um terminal
Pressione Ctrl + Alt + T
Execute:
exec sudo -i
apt-get update
apt-get install --reinstall aptitude deborphan
aptitude remove '?and(?reverse-depends(xubuntu),?not(?reverse-depends(?exact-name(gnome))))'
apt-get install --reinstall gnome
apt-get autoremove
apt-get clean
reboot