Well, this was an annoying one, and it’s hard to find the one thread that actually addresses it amongst the ones conflating it with an off-by-one error code (subtract one from 0x8004FF91. not going to actually say it here to avoid poisoning Google).
TL;DR if you can’t install Microsoft Security Essentials – even on a brand new install of Win7 64 bit – it’s probably due to Windows Update KB3004394. Uninstall that update, and MSE will install just fine.
UPDATE: KB3004394 has been acknowledged as bad by MS. And the problems are actually a lot more far-reaching than just MSE installs; the KB botched an implementation of root certificate checking that causes all code signing checks to fail. Affected systems (Win7 SP1 and Win2008 R2 SP1 at least) will not be able to install signed device drivers, will not be able to install MSE, will get unexpected UAC prompts in weird places (due to signed code suddenly appearing unsigned and therefore untrusted)… oh, yeah, and Windows Update will fail, meaning that they’ll have to be manually fixed by either uninstalling the bad KB (at which point Windows Update will work again) or by manually downloading and installing KB3024777. Repeat manually – you can’t get it from Windows Update until Windows Update actually works, so…
Get it all in one sock, Microsoft.
2 thoughts on “MSE Install fails with 0x8004FF91”
Thank you very, very much for this information. Unfortunately, I have this problem now on my Win7 x64 PC. And, having never needed to reverse a Windows Update before, I just used CCleaner this past week to remove all the files needed to uninstall those updates and service packs. I never chose that option until this week. Now the applet for uninstalling Windows Updates says I have no updates installed. Aaaargh!
Please, if you can tell me how to uninstall the update manually, or how to uninstall the problematic part of the update, please do so.
Thank you, in advance, for any help you might be able to provide.
Have you tried manually downloading and installing KB3024777?