Reinstallation from the ISO is the only way to be sure. Otherwise, you've plenty of settings to reset - this can be done for some packages on a per-package basis, but a reinstallation at this point would be the least painful and most effective method.
- Back up your home directory (to a USB key)
- Get a CD (burn your own from ISO, order one, put on another USB key) for the right version
- Install from scratch
- Put your home directory back
Note - you may find there are subdirectories you want to remove, such as .gnome, and let Ubuntu produce its own defaults, as you may also have unwanted settings in those.
Note 2 - If you've got server side stuff (/srv, or other things), you'll need to back up more than the home dirs.