Try BSPlayer. The current free version contains adware but you can get version 1.32.820 from
Old Versions. You don't even have to install anything. Just unzip the folder whereever, run bsplayer.exe, and drag your mpg into the player.
While it's running you can get jpg screencaps by pushing P for actual size or Shift+P for "what you see." The first time you might need open the preferences window (ctrl+p) to tell it what directory to put the screencaps into.