I don't think this will have anything to do with Windows as it is controlled by the email client. This registry edit will force it to display HTML.
I may not be all that clear on your problem, is it a problem reading an HTML email or are you not actually able to receive them which could be a security issue.
HKEY_CURRENT_USER\Software\Microsoft\Windows Live Mail\mail\Message Read HTML
1 - Read messages in HTML
0 - Read messages in Plain Text