Firstly I'd try a 'Detect and Repair' which is available from the Help Menu. You may need your installation disc if it is asked for.
If this does the trick, check for Office updates and install as necessary.

If this doesn't work I would try using the >>> Windows Installer CleanUp Utility <<< to remove Microsoft Office 2003.

Once installed, open the utility and scroll down to Microsoft Office 2003 and highlight it.
Click on the 'Remove tab'.

When it has run it's course, maybe a reboot and try reinstall Microsoft Office 2003 along with Office updates.