Photoshop not recognizing transparency?

I gave a friend some png files that have transparent backings. However, he recently had to install his photoshop over again because of an HD meltdown. Now when he opens up the pngs they all have a white background. Even if he removes the background and saves the file (or saves it as a gif or web file) and closes it, when he opens it again it will still have a white background. Is there anything we're not thinking of? A setting we forgot..? Something? Any ideas would be helpful.