I'll bet there is some program that was installed (like the free ones that come with cameras) that "stole" the rights to the file, then was uninstalled and failed to relinquish control (i.e. bad programming...)
So if you are in Explorer and click "View/Folder Options/File Types" and find the type of picture you are having problems with, and see what Windows is trying to open it with.
I usually change it to "Windows Picture and Fax Viewer" if something took over.
