viewing SOME jpg files

> Have a My Pictures directory full of folders containing
> photos from the last couple of years. Using Windows XP
>
> Never had a problem until today.
>
> Loaded some new photos from our latest holiday and
> thought I'd go back and have a look at some old ones -
> for old times sake!
>
> have filmstrip selected and on the folder icon - there is
> a preview of 4 photos from EVERY folder. When I click on
> 6 of the 16 folders, it says preview not available. The
> properties on the file show it as a jpeg file, and the
> default program is windows picture and fax viewer. I
> have always been able to look at these files before, as
> thumbnails, film strips and all sorts, but seemingly not
> today.
>
> for the new photos I loaded, and for lots of the old
> photos there is a preview/thumbnail/filmstrip ok.
>
> Can anyone tell me what might have gone wrong?
>
> thanks