Have you installed any utility which uses exiftool? See the links section of the exiftool home page for some examples. I suppose it is possible that one of these is launching multiple copies of exiftool. I'm not a Windows expert, but isn't there a way to tell what process spawned the exiftool process (as you can in Unix)? Then you could tell what was launching the exiftool copies. Otherwise, check all your startup processes. One of them must be launching exiftool.