If you have a Windows NT-based system (Windows NT, 2000, or XP), create a
separate Windows account for each person and, in each account, a mail
profile.
If you wish to use a single Windows account, create a separate mail profile
for each person and password-protect the individual PSTs (if you're not
using an Exchange server as your message store). Set up the profiles so
that Outlook asks which to use when it starts.