I have tried to select first Window Photo Gallery and then Adobe Photoshop 7 Editor as my Default Programs for jpg fiiles but I am still unable to open jpg attachments. I get a screen which says I typed it wrong!
Try the JPG download at the link below to restore the default settings and associations for jpg file to see if it may help. For now, are you able to save the jpg file to the desktop, then open it?
I spoke too soon! I am back to square one with jpg files. I repeated the selection of Windows Photo Gallery as the default and then restored the default settings - to no avail. I can save the attachment and then open it from the saved location.
If this did not start to long a go, then you may be able to do a System Restore using a restore point dated before this problem to see if it can fix this for you.