I think there is a way of passwording the .pst file that Outlook (not Outlook Express) uses. That probably stops the ,pst fiel being opened, much like any password protection of any file. Wouldn't be of any use though if you walk away from your puter leaving outlook running.
If it's the latter you are protecting against how about a passworded screen saver, or simply locking the computer [ctrl-Alt-Delete] (only available on win NT 2K & XP Pro)
*edit: beaten to it
