As per microsoft support site the following can be followed for backing up emails in outlook express
Copy Mail Files to a Backup Folder
To make a backup copy of your Outlook Express e-mail message files:
1. On the Tools menu, click Options.
2. On the Maintenance tab, click Store Folder.
3. Select the folder location, and then press CTRL+C to copy the location.
4. Click Cancel, and then click Cancel again to close the dialog box.
5. Click Start, and then click Run.
6. In the Open box, press CTRL+V, and then click OK.
7. On the Edit menu, click Select All.
8. On the Edit menu, click Copy, and then close the window.
9. Right-click any empty space on your desktop, click New, and then click Folder.
10. Type mail backup for the folder name, and then press ENTER.
11. Double-click the Mail Backup folder to open it.
12. On the Edit menu, click Paste.
13. Close the Mail Backup window.
For restoring the emails, see the link below:
Source: *support.microsoft.com/kb/270670