Outlook PST files quickly grow to epic proportions unless you take some actions to archive older emails. Archiving saves the older emails but removes them from your main Outlook file and keeps your environment easier to manage and running very snappy. Unfortunately the integrated archival in Outlook is not the easiest to deal with and since it removes all archived emails into a separate PST file you have to open your archive file to work with (and search for) older emails.
The Office Weblog has a thorough overview of a third party application, Mail Archival Pro (MAP), which expands the usefulness of email archival and makes the task very simple. The program gives the user complete control over the operation of the archival process and more importantly allows you to search and preview archived emails from within MAP. Head on over to The Office Weblog to see if MAP is right for your needs.