This isn't likely to be your new H/D rather you or someone has made changes to your software (most likely Outlook Express?) Have you recently updated it. Could also be a security issue as eml files can carry malicious code. New antivirus software or security update?
You might try in Windows Explorer going to View | Folder Options | File types and then go
to OE mail message (eml extension).
Click Edit and then Open and then Edit and in the application box paste the
following:
"C:\PROGRAM FILES\OUTLOOK EXPRESS\MSIMN.EXE" /eml:%1
and uncheck use DDE.