This problem is reproducible: installation progresses smoothly through language selection, system check, data folder setup, and database user setup. When the installation moves onto moodle/admin/index.php, however, I get an Error 404 from IIS, and can't progress further. I've experimented a bit with this, and have found the following data: if I remove the provided admin/index.php and create a fresh one, I can access it. I can access a phpinfo() test script in the same directory. If I rename the provided admin/index.php and try to access the renamed file, I can't access it. If I comment out all the contents of the provided admin/index.php file, I can access it.
I've never heard of a web server simply refusing to send a document just because it didn't like the document contents. In my experience, they usually send a list of parse errors or somesuch. I've even tried downloading and extracting a fresh copy, to no avail.
Has anyone encountered something like this before? I'm out of ideas, and getting a little desperate.