Never seen that. I usually have to set PS as the default for opening PNGs after a fresh install, but I've never seen PS unable to open PNGs.
If you need the plug-in, here's one that I KNOW works: http://tinyurl.com/n5ag65r
That's got the X86 and X64 PNG file format plug-ins in a zip.
Thanks Mark, that's exactly what I needed! I just replaced the old PNG plugin and it's working fine now. I really appreciate the help.