Modified Date used as Date Taken in Photo Gallery and Digital Imag

When I organise by date taken, for about 1500 of my 9000 images, both photo
gallery and digital image 6 are using the modified date as the date taken.

When I use explorer and properties, the date taken is present and correct
for these files (e.g. March 15th, 2005, 10:45am).

When I look at the properties through photo gallery or digital image pro,
the "date taken" is the modified date (e.g. 28th December 2007) and hence
when I organise them by date taken, these photos appear under December 2007
and not March 2005 as they should.

I have tried deleting the thumbs and metadata database files and when they
re-create, same issue.

Any thoughts on why modified date is being mapped to date taken?

Thanks, MArk.