OS: Windows 8.1 Enterprise x64
Primary browser: Opera (stable)

I know Flash is *supposed* to be included in all versions of 8/8.1, but it's my understanding that this generally only applies to Internet Explorer, not 3rd-party browsers. Most other browsers still need to have their own specific version of Flash for that particular browser.

I can watch YouTube, etc, in IE and Opera, but certain sites (in Opera) say I still need to install Flash and won't play videos until I do, I've done this before, it solved the issue, and Flash worked on all websites that support it. And I know these messages I'm getting aren't malware, popups/popunders, etc, I'm not an average user and know how to keep my PC clean and distinguish these kinds of things. Also, I don't want to install Firefox, Chrome, etc, just to watch videos on my favorite sites, anyone who suggests this will be dismissed out of hand and without consideration. Opera has been my main browser for years and Flash has never failed to install on any Windows OS/browser, up until now.

How can I resolve this? Is there a way to completely purge all traces of Flash from 8.1 and do a clean install? I've tried rebooting and then running the installer again, but it just gets to 90%, hangs there for a minute or so, then gives a failed message. I've ran an sfc /scannow, no corrupt system files found. All available Windows Updates are installed, drivers are up to date.

Thanks for any help!