Wow... I'm surprised that so many gaming nerds can rip on this guy for making something that is quite incredible.
I went to his site to see how he did it. He started by measuring all the basic lego part dimensions. From there, he found that Legos were proportioned very well to be combined in sizes to match computer components. He used
MLCAD to design the case. That program exports a list of components which can be used to buy from certain Lego parts sellers. He then bought all the parts (both Lego and computer), exported images from MLCAD to show how to build the sections, and set about building it.
He remarked that almost everything fit just as planned with very few modifications required.
I applaud the guy. That's an impressive amount of planning, creativity, and skill.