Try some cleaning up.
Do not archive mail on default OE folders. They will eventually
become corrupted. Create your own user defined folders for storing
mail and move your mail to them. Empty Deleted Items folder
regularly.
After you are done, follow up by compacting your folders manually
while working *offline* and do it often.
File>Work Offline (or double click Working Online in the Status Bar).
File>Folder>Compact all folders.
In Tools>Options>Maintenance: Uncheck Compact messages in background
and leave it unchecked. {N/A if running XP/SP2}.
In IE>Tools>Internet Options>Delete Temporary Files and include
Offline Content.
Finally, run a Defrag.
