Roady is absoloutly correct, and further to that, a brief note on PST files.

If outlook is installed on an XP machine it creates a storage file containing the mail and mail folder settings, mail rules etc. This is creted individually for each user when they first use Outlook.

So in your example, when you have instaled outlook on you XP machine in your profile. Log off, log on again as your users and open the program in each of their profiles so as to initiate the setup wizard that enables you set up accounts and rules etc (you'll probably want to do this rather than letting the young'uns struggle though it). This done each user will be running, effectively, the same application but be seeing dfferent mail in their personal PST file which is stored in their personal profile in windows XP giving them a little privacy from each other. You however will, as administrator, if you need to, be able to attach to their PST files (see outlook help system) for supervisory purposes as required.
Cheers
Dave.