You may need a tool to extract the messages, but try this first.
File | Identity | Add New Identity.
In the new identity: File | Import | Messages. Select Microsoft Outlook
Express 6 and Import /only/ the folder in question from the old identity. In
this identity it will have a new dbx file. Create a number of folders and
see if you can move messages out of the folder in this identity. Keep them
under 300MB.
If you are successful, you can either go back to the old identity an import
the new smaller folders back, or, if you are just archiving the messages,
leave them where they are and use that identity for archiving purposes.
How to Create and Use Identities in Outlook Express
http://support.microsoft.com/?kbid=209169
Tools for recovering messages if needed.
DBXtract:
http://www.oehelp.com/DBXtract/Default.aspx
DBXpress: {much faster for large files}
http://www.oehelp.com/DBXpress/Default.aspx
--
Bruce Hagen
MS-MVP Outlook Express
Imperial Beach, CA