it sounds like your file extensions have been changed. First you could try right clicking the picture and choose 'open with' then choose program. Windows will then give you a list of available programs that can view pictures. Click one and tick 'always slected program'. * the windows picture and fax view is the windows default viewer and it normally the quickest if you just want to preview the pictures.
To configure File extension in windows explorer goto - tools - folder options - File types. Scroll down to jpg and ensure the correct program is listed to open. I suspect that if you click the advanced button you will see from the actions box only printto is listed and the open command is missing. If it is click new - type open in the top box then in the bottom box browse for the program you wish to use for handling pictures.