If you would like to ensure that you always have a backup of your Outlook Express mailboxes available, there is an easy way to do it manually. Note that you must have enabled viewing of hidden folders (in Windows Exporer/tools/Folder Options/View Tab/ enable "show hidden files and folders") in order to successfully view your mailboxes.

Navigate to c:\documents and settings\(your username)\ Local Settings\Application Data\Identities\(some very long and complicated string of numbers and letters)\Microsoft\Outlook Express

You will see a .DBX file for each of your mailboxes. Transferring these to another hard drive or burning them onto a CD at regular intervals will ensure that you can always restore your email records should you be forced to reinstall Windows.

To restore your mailboxes in the event of reinstalling Windows, simply copy them back into the same folder once the install is complete, overwriting the existing files. You will now have access to all your email records.