Well if no one's going to try the WinRAR trick, I guess I shall have to reveal All.
The file is not Vista, not even a pre-release build. It's a meticulously crafted collection of files that really have no content apart from the same character repeated several times - open any of them in notepad and you'll see.
Compression works on a simple funda - find patterns, and then store the number of times it's repeated. So compressing a text file with, say, "Digit" written a hundred thousand times will create a text file that is most likely 50 odd MB, but when I compress it, it will be a KB at best - the compression algorithm will just read the content as "Digit * 100000", which obviously takes up little or no space.
And guess what? Every single file in this iso is like this.
WinRAR, incidentally, compressed the iso to 1.2 MB.
Finally, tech_mastermind: I hope you got your answer now. Someone could rename the goatse image to "Vista.iso", but that doesn't make it Vista now, does it? Please grow a few more brain cells before stepping out into the real world.