Have you tried running Sigil Portable from PortableApps? They are offering v 08.2, and older versions can be found as well.
http://portableapps.com/apps/office/sigil-portable
I have been running 32-bit Sigil Portable on win 8.1 64-bit for several versions with no problems. I don't really see any difference between running an "installed" versus a "portable" version. I even set it as the default epub handler in Windows.
If you haven't tried that, give it a shot.