Look in:
HKEY_CLASSES_ROOT\MIME\Database\Content Type\application/msword
It should contain a single string value, called 'Extension', containing '.doc'
Then, go and look in:
HKEY_CLASSES_ROOT\.doc
It should have a string value, called 'Content Type', containing 'application/msword'.
The '(Default)' value should contain 'Word.Document.8'
Then, there's a bunch more scary stuff in:
HKEY_CLASSES_ROOT\Word.Document.8
The easiest option, however, is probably just to go to Control Panel/Add or Remove Software/Microsoft Office, and try the 'Repair' option.
Whether that actually does any good remains to be seen, however.
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roger