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.