Unless you've removed Outlook Express and are using some other email
program, eml files will open natively in Outlook Express when you double
click them.  There is no external viewer, although you could right click on
one and select Open With Notepad (just do not put a check mark to always
open files of this type.........).

OE should view the file as e-mail when you doubleclick on the file.
But any textviewer will view eml-files.
Start wordpad, click file -> open .....

Grtz,

TR