See my reply to your slightly newer post.

General precautions for Outlook Express:

Do not archive mail in default OE folders. They will eventually become
corrupt. Create your own user defined folders for storing mail and move your
mail to them. Empty Deleted Items folder regularly. Keep user created
folders under 100MB, and Default folders as empty as is feasible.

After you are done, follow up by compacting your folders manually while
working *offline* and do it often.

Click on Outlook Express at the top of the folder tree so no folders are
open. Then: File | Work Offline (or double click Working Online in the
Status Bar). File | Folder | Compact all folders. Don't touch anything until
the compacting is completed.

Turn off e-mail scanning in your anti-virus program. It is a redundant layer
of protection that eats up CPUs and causes a multitude of problems such as
time-outs and account setting changes. Your up-to-date A/V program will
continue to protect you sufficiently. For more, see:
http://www.oehelp.com/OETips.aspx#3

In Tools | Options | Maintenance: Uncheck Compact messages in background and
leave it unchecked. {N/A if running XP/SP2}.

And backup often.

Backup & Restore:
http://www.insideoe.com/backup/

This is a great two click program:

Outlook Express Quick Backup (OEQB)
http://www.oehelp.com/OEBackup/Default.aspx