Outlook Express is part of Microsoft office. Try runni9ng the Office CD and do a repair of the installation. If that doesn't do the trick, try reinstalling the Office application.
Edit to remove the errant '9'. Ah screw it.
Actually Express is not Office, it comes pre-installed in XP. Plain outlook is Office.
Check the shortcut, it sounds like the Express shortcut got remapped to something else. It should go to c:\program files\outlook express\msimn.exe