I'd say the .jpg file you're trying to view isn't quite conforming to specifications, i.e. is somewhere between "OK" and "corrupted." Assuming that Paint and Windows Photo Viewer have slightly different ways of decoding the .jpg file, I'd imagine that one can see it just fine and the other can't.
It would be interesting to know where the .jpg came from/which program created it. Maybe try a different program instead - try Googling for software to fix problematic .jpg files as well.