Usando este comando apt-get install xubuntu-desktop eu recebo isto:
Some packages could not be installed. This may mean that you have requested an impossible situation or if you are using the unstable distribution that some required packages have not yet been created or been moved out of Incoming.
As informações a seguir podem ajudar a resolver a situação:
The following packages have unmet dependencies:
xubuntu-desktop : Depends: gtk2-engines-pixbuf but it is not going to be installed
Depends: xubuntu-artwork but it is not going to be installed
Depends: xubuntu-default-settings but it is not going to be installed
Recommends: bluez-cups but it is not going to be installed
Recommends: cups
Recommends: gmusicbrowser but it is not going to be installed
Recommends: gnome-system-tools but it is not going to be installed
Recommends: hplip but it is not going to be installed
Recommends: pidgin but it is not going to be installed
Recommends: printer-driver-splix but it is not going to be installed
Recommends: xchat but it is not going to be installed
E: Unable to correct problems, you have held broken packages.
Os erros relatados são devidos a você não ter o repositório Universe habilitado em suas fontes de software.
Felizmente, temos um ótimo Q & A que aborda isso em detalhes - não vou repetir as mesmas informações aqui:
Depois de ativar o repositório universe, execute novamente o processo de atualização:
sudo apt-get update
sudo apt-get install xubuntu-desktop