Installing KDE in Ubuntu
KDE applications or KDE as a whole is just a matter of downloading, installing and updating more programs, which means bigger download sizes, more disk space requirements, and more updates. There is also one more thing to keep in mind: when you install KDE on Ubuntu (or better, on a Gnome environment) your menus will be populated with both KDE and Gnome programs. In addition, you will be able to see and run some programs in KDE but not in Gnome (and vice versa). If you are comfortable with all these, then you can start installing KDE immediately.
Go to System -> Administration -> Synaptic Package Manager and enter your password when prompted. In Synaptic, search for kubuntu-desktop and tick the box to select it. You will be presented with a list of additional dependencies which you will need to install in a dialog box. Click the “Apply" button on the dialog box and then again click on “Apply" in the main Synaptic window. The files will be downloaded and installed on your computer.
When the installation is complete, you will be asked to select your default window manager. If you want to stick with Gnome and switch to KDE when you want, select “gdm." If you want to use KDE as default select “kdm." If you change your mind later, open up a terminal and issue the command sudo dpkg-reconfigure gdm. Change your default desktop manager and follow the steps on the screen.
In the log-in window, you can go to Options -> Select Session and choose whether you want to log in to Gnome or KDE. If you select “Default Session" in this screen, you will log in to the desktop environment, which you have set as default as discussed in the paragraph above.