PCWorld shows how to test-drive the 'Big Three' desktop environments on Ubuntu Linux and switch among them at will.
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Several different permutations of the problem, yes, but a single, simple answer to all. Here, I'll lay out one not-too-complicated set of instructions for adding any of the “Big Three” desktop environments to any flavor of Ubuntu. Most of these steps happen at the command line, so first open a Terminal window, by clicking Applications, Accessories, Terminal. Kubuntu users, open a Konsole window via System, Konsole in your start menu. (Ye gods, how I hate the endless stream of K-named apps in the KDE universe.) Now let's get down to brass tacks.