This kind of thing is not uncommon and its other variant is one Office
program will invoke the installer for another, of the installer for Omni
Page Pro if you've ever had that on the box.
I think you're best bet is to definitely use the WICU Colin invoked, which
is a little cleanup utility that is very helpful in any large MSFT program
uninstall and subsequent install--it does no app uninstalling but it will
get corrupt files, .dlls, reg strings out of the way of the MSI the Windows
Installer. The Office setup teams use it all the time with problems like
this one. You certainly could try a "detect and repair" repair of Office
located at the Help menu if its 2003 or earlier or Add/Remove and that may
do the trick. If that doesn't then...
Uninstall your current Office and reinstall and you might consider Office
2007 trial if you want to get a look at some of the new Office features
(Beta on Beta so what I've been using it for good while). MSFT has plenty
of instruction on Office 2007 on its site including videos.
Description of the differences between repair and reinstall in Office XP and
2003
http://support.microsoft.com/kb/298027/en-us
Description of the Windows Installer CleanUp Utility
http://support.microsoft.com/?scid=kb;en-us;290301
Let us know how you make out.
CH