The problem is it could be so many things
to me it sounds like it might of
got corrupt some how? try doing a detect and repair. To do this go to start >
Control Panel > Add/remove programs, select office 2003, and click on CHANGE.
It should now display a maintenance page, select detect and repair > next.
Follow the on screen instructions and see if this resolves the problem.
If not you can try removing and re-installing Outlook 2003 but make sure you
back up your files i.e .pst files etc.....
Please let me know first on you get on with the detect and repair.