This is usually caused when two versions of Office were installed, and one was uninstalled after the other was installed. This creates a mismatch in the library's the applications use to function correctly. (Normally you should only install a newer version AFTER the old one was installed to insure the correct pointers exist in the registry.)
Step One: Launch Control Panel
Step Two: Select Programs and Features
Step Three: Select the Microsoft Office currently installed
Step Four: Run Repair