[fung0]

That seems to have worked! After all this mucking about, I'm not jumping to conclusions, but Sigil 0.7.2 does seem to start normally now. Hopefully, it will keep doing so!

My firewall, Comodo, alerted me that just before I saw that DHtmlHeader message, the Sigil installer called VCredist. So, the message is presumably coming from Microsoft's Visual C component updater. Does Qt need that? If so then I'd say the problem is that Qt is leaving crud in the Registry, that trips up later installs. Oddly, my Registry now has no 'Trolltech' entries at all. Maybe I have other Qt apps on the system that are interfering with each other? Or could the Trolltech entry be connected with an older version (or different build) of Qt? (Hopefully, I'm not going to find that something else has now stopped working!)

I also emptied my Temp folder before reinstalling this time... could be a previous version of the installer components got stuck in there. Anyway, I seem to be back in business. Amazing.