Hi, if you have a computer with lower specification (between 128 -256MB RAM), you may find that KDE and Gnome desktop-environment in Ubuntu are too slow on your computer. There is an another desktop-environment called XFCE which is relatively lighter weight.
To install XFCE desktop environment in an already existing Ubuntu/Kubuntu paste this command in Terminal without quotes
" sudo aptitude update && sudo aptitude install xubuntu-desktop "
and restart the PC for better measure. In the login screen use session and select Xubuntu