I currently have Ubuntu 13.10 installed dual-boot with Windows 7. I want to install Xubuntu 13.10 over Ubuntu 13.10 without messing up with Windows and without losing my data in Ubuntu so: is there a way to install Xubuntu over ubuntu and keep all my data and all my programs? or do I have to uninstall ubuntu ( and lose all my data) and then install xubuntu?
You just have to install the xfce desktop:
sudo apt-get install xubuntu-desktop
If after installing xfce you want to uninstall Unity, you need to follow the specific instructions for your ubuntu version.