Ubuntu uses Gnome window manager as default desktop.If you want to install KDE desktop,which would be a lot more familiar to Windows users.Here’s an easy way.
Run following command in terminal:
sudo apt-get install kubuntu-desktop
It will take a while downloading updates,then you will be prompted to select the login manager.
When installation finish,reboot your computer.When you get the login window for typing usrname and passwd,click on Options button and select the session (KDE) as you want.
You also can switch between Gnome and KDE in this way