You already have installed Ubuntu 11.04 but you haven’t liked the Unity. You want to try another window manager like KDE. So you can install Kubuntu, it comes with KDE.
To install Kubuntu, open a terminal and run:
sudo apt-get install kubuntu-desktop
During the installation, you will be asking about which display manager you want to leave as default. Choose kdm.
Now, if you want a full KDE installation that include multimedia, networking, graphics, education, games, system administration tools, and other artwork and utilities, still in the terminal, run:
sudo apt-get install kde-full
After that, restart your computer and in login screen, don`t forget to select KDE