Evidently, there's something wrong with your Office installation. Try repairing the Office installation (via Windows Control Panel > Programs > Programs & Features > Microsoft Office (version) > Change > Repair).

Unless the 'Reset All' option is greyed-out (because you don't have any custom shortcuts), you can also restore the keyboard shortcuts to their original settings by clicking 'Reset All' at the bottom of the Customize Keyboard dialog box. See under File|Options|Customize Ribbon>Keyboard Shortcuts:Customize.