I solved the problem for Windows 7 64 bit users and Vista 64 bit users
- Open up regedit.exe (The windows registry editor)
- Navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Microsoft Internet Mail Message WLMail\shell\open\command
- Change "C:\Program Files (x86)\Windows Live\Mail\wlmail.exe" /eml:%1 to "C:\Program Files (x86)\Microsoft Office\Office12\OUTLOOK.EXE" /eml "%1" (do not forget the double quotes!)
- Restart your computer
This way, windows thinks that "Windows Live Mail" is still the default program for opening .eml files. However, we changed the registry so that instead of opening Windows Live Mail, we are opening OUTLOOK.EXE. What I mean is that if you opened the control panel, clicked on Default Programs and associated .eml files with Windows Live Mail, we've tricked out the registry so that it will open outlook instead--even though Windows still says it is associated with Windows Live. Confusing, I know.
The best analogy are those old commercials where "We've secretly changed Lisa's favorite detergent with new all temperature Cheer!" In this case we secretly changed Windows Live Mail with Outlook when it comes to opening .eml attachments.
Anyhow, this trick worked for me. I can open .eml files just by double-clicking on them. You can always change things back if you decide to switch to Windows Live.