Trouble Opening PNG Extension Files?

I'm currently running on a Windows 7 operating system and my computer was just purchased less than six months ago. I am unable to open .png extension files (these are images). They should be opening with Windows Photo Viewer, like all my .jpeg extensions do, but instead I get an error message: [filename] is not... show more I'm currently running on a Windows 7 operating system and my computer was just purchased less than six months ago. I am unable to open .png extension files (these are images). They should be opening with Windows Photo Viewer, like all my .jpeg extensions do, but instead I get an error message: [filename] is not a valid Win32 application. On any other system, I can open .png files just by double-clicking, but for some reason my computer is different. When I first got my computer, I was able to open these, but now something has changed. I tried changing the preferences of which program the computer should use to open this file. There is a check box that says something like "always use this program to open this type of file," but it is greyed out and I am unable to check it.
Also, before anyone says this, when I right-click it and go to properties, I CANNOT just click "change" and change the program with which the file opens, as this option does not appear. Also it is NOT a virus, and I should not have to restore my system. I assume this just has something to do with the registry.
Any legitimate help would be appreciated.
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