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You installed Ubuntu, and since it's so darn configurable, you decided to try a whole lot of different settings and configurations. Maybe to the point where things just aren't as they should. How do you restore your desktop to it's original state without re-installing?

If for any reason you need to restore your desktop to its default state, open a terminal and run these commands:

cd /home/username – This moves you to your user's /home directory, (which you should automatically be in when you open a terminal anyway) – be sure to put your actual username instead of /username.

Then:
rm -rf .gnome2 .gconf .gconfd .metacity – This command erases/removes all your gnome-desktop configuration files, forcing a re-spawn at restart. It's safe and effective.

To simply restore the top and bottom panels to their original state, use the following command:
gconftool --recursive-unset /apps/panel && killall gnome-panel

Here's an Ubuntu Forum thread discussing the procedure: http://ubuntuforums.org/showthread.php?t=1060406

On a similar note, when installing Ubuntu or any other Linux OS, if you take the time to install a separate /home partition, restoring can be done at a deeper level.
Essentially, if you've installed a separate /home partition, you can re-install the entire operating system without over-writing your folders, documents and other important data.

Here's a link to Installing Ubuntu with a separate /home partition.