Faced the same Problem with Outlook2003 and Win7 Pro / 64bit.
None of the proposed Solutions found on the Web worked for me.
Finally I had to set a registry key manually:
HKEY_CLASSES_ROOT\SystemFileAssociations\.htm\Shell\edit\command
with the Standard value set to:
"C:\Program Files (x86)\Microsoft Office\OFFICE11\winword.exe" "%1"