Vista - pixel aspect ratio correction; .jpg glitch

Vista seems to be radically distorting .jpegs.
This issue is inconsistent. I have many hundreds of photos stored as .jpgs
and they seem to dsplay fine, as thumbnails, previews, or when opened into
photoshop. But I also have 100's of illustrations and sketches, stored in
..jpg format, and many--not all [lets say: 1 in 5] are being viewed in Vista
thumbnails and previews as squashed in the horizontal axis to just a few
pixels wide.

When they are opened into photoshop, equally distorted (although, the
thumbnail and preview of the image in Photoshops Browser looks correct), I
get a message that says:"Pixel aspect ratio correction is for preview
purposes only. Turn it off for maximum image quality."

I never play with pixel aspect correction. All these images had square
pixels when they were created, and do so currently, on the computer
(WinXPpro) that they came from.

So, how does this happen, and how can I fix it, in Vista?-- without browsing
for and opening every single image in photoshop and correcting it and
resaving it.
--
Rob
Lead Artist