The program can't "revert" without uninstalling W2003 and re-installing
W2002. a) this can't have happened without your noticing *something* (like
when it asked you to insert the CD); and b) saving an email wouldn't trigger
such an event.
In any case, there's no difference between W2002 and W2003 as far as file
format goes.
What are the symptoms of the problem?