On the official diablo website, if you haven't seen it already, this is a solution I think might just make it work. For both mists of pandaria beta and diablo 3, it kept stopping at the 'updating setup files', and didn't go any further.

A guy on the official forums gave this solution, just follow what it says, download the two files (for the second one make sure you select your region). And then trying launching the instillation. (obviously make sure all other programs are closed before trying)

This solution fixed it for me, I'm now downloading mists on my new pc, and going to try diablo tomorrow.

http://patchup.info/d3/tools/Win/