Moving Outlook Files

I support a small network of about 25 users. It is network without a domain running Sun Solaris 7 server,windows NT workstation, 2000 pro,Windows 98 and XP.We don’t have an exchange server(we have a service privider that support our e-mail).We recently had some new computers running windows 2000 pro with Office XP intalled on it.Could someone help me with the best way to migrate outlook mails from one computer to the other.In other words migrate old e-mail to the new one.Thanks