If you want Outlook to prommt your users for a separate password from the network login, this might be what you want:
In Outlook, right-click on "Outlook Today [Personal folders]
and select properties. On the General tab for Personal Folders Properties box select "Advanced." When the Personal folders box pops up, select "change password". You'll be prompted to put in the old password (blank if none), the new password, then once more to confirm. Close all windows incl. Outlook, then reopen Outlook. You will be denied access w/o entering the password you set earlier. NOTE: the first time you use the new password Outlook will prompt you to input it twice; thereafter only once is necessary.
Note that the password you set will be attached to the .pst file for the user and not just the machine (if you store .PSTs on a central server for backing up) So if you open the e-mail from a dif. machine, the same password will be in effect.
Don't forget the password! It's a pain to crack into Outlook w/o it....
Good luck,
Matt