Tag libraries are stored in the Dreamweaver configuration folder. When you make changes to the default tag library, the changes are stored as .vtm files in subfolders of the TabLibraries folder of your personal configuration folder.
To restore the default tag library, delete the TabLibraries folder in your personal configuration folder. Caution: do not delete the TagLibraries folder in the main Dreamweaver configuration folder (the one in Program Files on Windows or Applications on a Mac).
Details of how to locate your personal configuration folder can be found in http://forums.adobe.com/thread/494812. For further information on the tag library file format, see http://help.adobe.com/en_US/dreamweaver/cs/extend/WS5b3ccc516d4fbf351e 63e3d117f53d77c2-7ffe.html.