Simply run MiTeC Mail Viewer, point it at the location of the file, and then select the folder you want to view. You'll then be able to browse through the e-mail, and save individual files as well as attachments. It's also nice that you can view not just the e-mail itself, but also the source code as well. This can come in handy for tracing suspicious e-mail or checking for phishing scams.
The program does have some problems, though. It didn't properly read HTML messages; I could decipher the content, but it didn't look like the original message. Aside from that, MiTeC Mail Viewer is free and simple, so worth a try if you need to view Outlook Express e-mail without having access to Outlook Express.