Ever run into a situation where you want to insert another document or file, using Insert->File, into a Word document, but the document type isn't listed?
Chances are, the proper file for converting that type of document is missing from the \Program Files\Common Files\Microsoft\TextConv file.
Had a user that was missing several of the files, but in particular, the excel32.cnv was not there, which meant that she couldn't insert Excel workbooks into Word. Insertion of Excel workbooks into Word as "objects" still worked, however, as this is done through a different mechanism.
Anyhow, the fix was to copy the excel32.cnv, plus any other missing converters, from a working system to the problem system.