If you have set up some keyboard shortcuts and then realize you have overwritten other shortcuts, you can remove the new shortcuts or reset the old ones. The Reset All and Remove buttons are also available in the Customize Keyboard dialog box.
The Reset All button resets all of the keyboard shortcuts to the default ones that came with Word. You should only use this button if you are happy to get rid of all the keyboard shortcuts you have set up yourself.
The Remove button allows you to remove particular shortcuts. Simply select the shortcut you want to remove and then click the button. This is the button to use if you want to get rid of a few shortcuts but not reset all of your personalized shortcuts.
If you remove a key combination that you had set up to use one of Word's built-in (default) shortcuts, the key combination will revert to the default shortcut.