Back up all the Outlook data files. Then do the uninstall and install 2003, finally, copy the files back. You may need to import the data from the various files, not sure.
Here's an article with details of the file locations:
http://itinfo.mit.edu/article.php?id=6972
There's lots more such articles out there...
Here's a useful one from MS:
http://support.microsoft.com/kb/287070
SD