Reset the file type association for .eml to Outlook Express. It's easy:
- save such an attachment
- open Windows Explorer
- find and select the saved file (either you see the .eml extension, or an Excel logo)
- right click, Open With ...
- from the list, choose Outlook Express or browse to the program (it's msimn.exe in c:\program files\outlook express)
- CHECK the box 'Always use this program to open ...'
- OK

Hope this helps,