Unexplained .jpeg format

Hiya, One of the machines here, when you open a folder with jpeg files, for some reason it shows them in acrobat format. Now I have gone into folder options and try changing it to jpeg format, but it doesn’t work.Only way I can view these files is by dragging them on to imaging. But how can I change it back to jpeg format where when you double click it, it will open up in Internet Explorer screen. Please help