WinBoard is the best thing I know of for the job. It can load and save whole games (e.g. using PGN files) or individual positions (e.g. using FEN files).
The installed version of WinBoard is 1.5 MB, but I have just run an exe packer on it on my computer and got it down to 0.5 MB. If you would like me to send it to you, just let me know.