Microsoft Security Essentials Install Error 0x80070643

While trying to update Microsoft Security Essentials to the newly released version 2, the installation failed with the remarkably descriptive error 0x80070643. Looking for a solution on the web was no help either.

So I tried to solve the problem by uninstalling the old version. Installing all critical and optional updates via Windows Update. Rebooting multiple times while doing so. Deactivating Microsoft Defender’s real-time protection. Sacrificing two goats and finally handing over my soul to the devil. But nothing worked.

At the end I got it to work. However, I changed two things at once before trying to install MSE2 again, so I’m not sure which one of those changes actually fixed the problem. But here you go:

• STOP the “Windows Defender"-service (I don’t think that solved the problem)
• START the “Windows Firewall"-service (But this one did, I guess)

As I don’t use Microsoft Firewall (or any other firewall software), I didn’t just deactivate the firewall but stopped the service altogether. That seems to have bothered the installer.

Now MSE2 is installed and working properly and with that: the option to opt-out of SpyNet.