It does sound strange. However, I did get the file from the link on the main Sigil page,
https://code.google.com/p/sigil/. (Just tried it again, and the files seem identical.) I am not running the installer from my USB drive... it's on my D: drive, which is a 1TB Caviar Black, and installing to my C: drive which is a SanDisk SSD. I even tried installing with the USB drive unplugged, but Sigil still didn't work.
A DHtmlHeader does sound like something that either Sigil or Qt might want. I can't help wondering if something on my system isn't tricking the installer into this behavior.
I've been searching my Registry for Sigil entries. Nothing too unusual... a tiny entry for the program itself, several entries in the audio policies, a bunch of entries to link Sigil with epub files. However, there is a larger entry for Qt, under the heading Trolltech. I'm next going to try uninstalling Sigil and then deleting that one before installing again.