I left my Windows 7 x64 PC to do a backup overnight and when I looked at it in the morning, there was a pop-up saying I should go online to find an antivirus and enable windows defender. Turned out that my Windows Security Essentials 2.0 was no longer running and after a restart, all traces of it were gone. I had it installed for several months now and did nothing to uninstall it. I reinstalled it, but it would be interesting to know how something like this happens, particularly to an antivirus program. The computer did not restart during the night, so this shouldn't have been due an update (last update was installed on July 22).
UPDATE: Since I can't answer my own question, here's what I just found in the event log:
HRESULT:0x80070643 Description:Cannot complete the Security Essentials Upgrade. Security Essentials is not currently monitoring and helping to protect your computer. Please restart your computer and try again. Error code:0x80070643. Fatal error during installation.
Here's more: http://answers.microsoft.com/en-us/protect/forum/protect_start/microsoft-security-essentials-mse-disappeared/b4ca26b3-d3ef-465e-89d0-37711f0da2f8?page=1&tab=MoreHelp
So hopefully this wasn't a virus.
Some viruses and SpyWare have been known to disable or remove security software (like Anti-Virus and Anti-SpyWare programs) as part of their installation so they can operate without interference.
In your case, I think there's a good chance that you've encountered of these viruses or SpyWare that specializes in removal of MS Security Essentials.
You'll probably need to scan your hard drive from a clean machine (a computer that's not infected by viruses or SpyWare) to catch the infection while it's dormant.
I had a similar experience (only on 1 of my PCs) and think it was caused by the update below. The usual restart and run Win update seemed to cure it
Microsoft Security Essentials Client Update Package - KB2544035
Installation date: 06/08/2011 09:09
Installation status: Successful
Update type: Important
This package will update Microsoft Security Essentials client on the user's machine.
More information: http://go.microsoft.com/fwlink/?LinkId=195462
Help and Support: http://go.microsoft.com/fwlink/?LinkId=195463
I had the same problem, but could not re-install. It just failed with error code 0x80070005, and I also had the error you mention (0x80070643) in the logs.
To be able to re-install, I had to first delete the whole folder as described here:
rmdir /S /Q "%PROGRAMFILES%\Microsoft Security Client"
Other people (like Mark Russinovich) could solve the re-install problem by deleting the
registry key, and possibly deleting a bunch of other Security Essentials related keys.
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