Cause
The installation of Word is damaged.
Solution 1
Use repair tools to fix the Office installation:From the Windows Control Panel: On the Start menu, click Control Panel. Then, click Uninstall a program. Click the version of Microsoft Office in the list of programs. Click Change. Select Repair and continue with the assistant. Restart your computer.
Solution 2
Reset the user options and registry settings in Word using Microsoft's Fix it tool: http://support.microsoft.com/kb/822005/en