If you are really desperate, you could try using
PAQ or
7-Zip, although I make no guarantees of whether you will achieve what you want or not. Not to mention that the compression algorithms are very memory and time intensive, and only god knows how long it will take to compress.
Then of course there is the whole fact that these aren't very popular compression programs, and you might forget what program you used. But that is a risk you will have to take.
Before trying all this though, Windows includes an excellent compression tool in the command prompt called "del," which I think you should try. Just type in "del" and the directory and file name of what you want to compress, and boom! tons of space saved.
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