What kind of files are they? If you cannot see the file extensions on them (for example: my_image.jpg; where the ".jpg"
part is often hidden from your view), please right click on the file and select Properties to determine it.
Also, which version of Windows are you using?
I had something sorta kinda like this happen to someone who received a *CMYK* .jpg file, and their version of Windows (Win2k *if I recall correctly*) was unable to view it. If the file(s) are indeed image files and *not* native PowerPoint files, then perhaps we could suggest an alternative image viewer for the OP that could view and convert the file(s); in much the same way I often just tell folks to download VLC to play problematic media files.