Creating a "Custom Archive" in Outlook

I have a client who's outlook has become very large and is causing his computer to slow down. I told him he should archive some of his mail to reduce the load that outlook is handling. He wants to know if there is a way to archive individual emails into an archive, as opposed to archiving entire folders. He has everything sorted into folders now. He would like to keep the folder structure and simply move many of the items from the existing folder structure to an archive structure. I have never heard of this. I usually just archive by date. Can this be done in Outlook 2003, and if so, exactly how. Please be specific. Thank you.