What happens when you double-click on the files? You need an image editor or viewer to read PNG files. I'm not sure when support for PNG was added to MS Paint, but I think it's available in XP and later. If nothing happens when you double-click, or you get a message asking what program to use, try right-clicking on the file and choosing Paint or Windows Picture Viewer. Of course, I've you've got another picture viewer/editor installed, like Paint Shop Pro or PhotoShop, use that.
Dunno about Macs.