An alternative to using a full desktop such as Gnome or KDE is using a window manager. This is a program that controls the placement and appearance of windows on the display. For instance, take a look at an open application. You'll notice that, depending on what desktop you're using, you should have a panel along the top that includes several buttons to click on - minimise, maximise, close and so on. The window manager is responsible for drawing these.
Gnome, KDE and Xfce include their own window managers as part of the desktop to ensure a consistent look and feel - Gnome uses Metacity by default (although Compiz will usually be enabled instead in Ubuntu, so you can have the cool effects), KDE uses KWin, and Xfce uses Xfwm4.
There are a number of other window managers that you can use on a standalone basis, without Gnome, KDE or Xfce. These generally lack the visual appeal of something like Gnome or KDE (with the possible exception of Enlightenment), but achieve better performance, making them ideal for older, less powerful machines.
There are a number of these - Fluxbox, Openbox, aewm, the list goes on. If you're interested in looking, try searching for "window manager" in Adept or Synaptic - you'll find a load of them available from the Ubuntu repositories. If you decide to install one, a word of warning: you'll have to do the configuration yourself. It's not that hard, but not all of them have a graphical configuration tool, so you may need to edit a text file and do other work from the command line.
One very popular one, which I have installed alongside KDE, is IceWM. This window manager is laid out in a similar fashion to Windows, making it an ideal choice for those who are used to Windows. It's easy to theme it, too - you can find plenty of themes, some based on Windows XP or Vista, and others completely different. Check out this link for a very handy tutorial on how to set up IceWM (please note, this isn't terribly up to date). I'd recommend you install the icewm-themes and menu packages as well, and ignore the icemc package as that no longer seems to be in the repositories. Then, once you've logged out from your existing desktop, log back into IceWM from the login screen (you'll need to change the session type in this screen, there's a pulldown menu for it), then open a terminal and enter the following:
to update the menu in IceWM with your installed applications.
If your computer is a little sluggish with your current desktop and you're not too bothered about a fancy desktop, it's worth considering a switch to a window manager. It's a little more work than the default desktops, but may well be worth the tradeoff. For my money, IceWM is the easiest to use, but Fluxbox is also good, and Enlightenment is very promising (if E17 ever gets released!).
One of the reasons for IceWM's popularity is that it's easy to theme it. Here's an example - the FauxGlass theme from Box-Look.org:
I can't deny that IceWM isn't the prettiest desktop available, but it's fast and practical. If your computer is a bit old and slow, or you're just not happy with the speed of your Linux desktop, IceWM is a great alternative to Gnome or KDE.