Have you encountered this behavior in any applications other than Microsoft Word? If it is only in Word you can spend hours debugging, but if it were me, I would uninstall Microsoft Office using the uninstall utility on the install CD or downloadable from the Microsoft web site. Then re-install Office and apply any patches you find for Office on the Microsoft web site. It doesn't take that long to do.
If that does not solve the issue, then start looking in Activity Monitor for other applications/utilities that may be running. Also assuming this is a USB keyboard, I would look closely at any USB device drivers you have installed.