Indexing contents of TIF files

I'm using Microsoft Document Imaging (from Office 2007) to create searchable
TIF files of my documents. (MDI performs an OCR and embeds the text within
the file.) When I was running Windows Desktop Search under Windows XP,
these files were indexed and showed up in the search results. However, the
search feature under Windows Vista seems to miss these files. When I look
in the options for the search feature, both TIF and TIFF files are marked
for indexing both the file name and the contents. How can I get Windows to
include the contents of TIF files in the search?

--
Michael