Photoshop CS3 Not Opening .png

While trying to open various pictures from "My Pictures" which were saved a . png, I noticed that the pictures would not show up as a thumbnail. So I went down to see if I had a filter that only shows . jpeg or whatever, but I see that it says "All Formats". So I resorted to typing in the file name to open it. After I clicked open a notice came up that told me that Photoshop "Could not complete your request because it is not the right kind of document" (exact quote from the pop-up). it seems to be only . png that it will not open, . jpeg, . tiff, . psd, etc. are all working, only . png will not show up.

Please if I could get some feedback for this problem. I would greatly appreciate any replies or information as to why this is happening.
~Thank You

Are you sure they're PNGs? Just because they have a PNG extension doesn't make them PNG files.

Bob

I'm pretty sure they are, but you see I don't even get the option to open a .png file (which I'm sure is a .png file) so even if these pictures weren't a .png file I still can't open other PNG files in the first place