What Gordon means is to create separate accounts in an NT-based version
of Windows under which each account has its own profile directory where
the application data files get saved and have permissions to prevent
other accounts from normally accessing them.

If you don't want to use separate accounts, one for each user, then use
mail profiles (see the Mail applet in Control Panel).  You have an
indefinite number of mail profiles per account and each mail profile can
have a different set of e-mail accounts.  If you select a mail profile
as the default, that one gets used without a prompt when you load
Outlook.  If no mail profile is designated the default then you get a
prompt when you load Outlook to select one.  Although you can password
protect PST files (i.e., your message stores), you cannot password
protect mail profiles so anyone using the computer under your login can
load your mail profiles (but will still get prompted if you password
protected the PST files that are used by your mail profiles).  Because
of the requirement to select a mail profile when Outlook loads, you need
to exit Outlook to change mail profiles.  Mail profiles were probably
provided to accommodate Outlook users on 95-based versions of Windows
where there is no security and equates to identities in Outlook Express.

Since you have Windows 2000, create an account for each user.  They each
get their own profile path (%userprofile%) where each has permissions
(if you use NTFS) that deters users from accessing each others data
files under that profile path.  There are no permissions when using the
FAT file system (but you still do get separation of application data
into different user profile paths).  Use Windows 2000 the way it was
designed to be used.  Use accounts.