Normally, when you uninstall Dreamweaver or the Extension Manager, your personal configuration folder is left intact. This is designed to preserve your preferences when reinstalling on the same computer. However, if the reason you're reinstalling is because the program has started acting erratically (or not at all), you should also delete your personal configuration folder before reinstalling.
In fact, it might not be necessary to go through the process of uninstalling and reinstalling. Quite often, deleting the personal configuration folder does the trick, and resets everything back to the factory defaults.
Instructions on how to locate your personal configuration folder. The Extension Manager has a configuration folder of its own in the Adobe folder two levels higher than the Dreamweaver configuration folder.