I've been using the Microsoft Security Essentials for a couple of months now, as it has the lowest impact on the machine.
As I'm a power user, I'm very careful about what I do, having security in mind all the time. But it seems something slipped in, and I noticed that Microsoft Security Essentials stopped loading. Google-ing around, it seemed that all the Microsoft Security Essentials services were disabled. Enabling and starting them, they would stop and become disabled again in seconds.
Trying a couple of things:
1. Malwarebytes' Anti-Malware: Restart in save mode with networking, run update and full scan. Nothing.
2. Microsoft Security Essentials: Restart in save mode with networking, run update and full scan. Nothing.
3. McAfee Stinger: Download latest version, run full scan. Nothing
I then made peace, a format was in my near future.
Then I noticed this story on MyBroadband:
"AVG 2012 Released in South Africa" - http://mybroadband.co.za/news/software/33812-avg-2012-released-in-south-africa.html
Yes, very strange it was yesterday, must be that I know somebody?
With nothing to loose, downloaded and installed. Then remembered should probably uninstall Microsoft Security Essentials, and did so. Did not even restart, ran the updates for AVG, did the full scan, and it found a "unknown thread" in software I had in my DropBox from years ago.
Had it clean it up, and all seems well.
The moral, when you are ready to format, give another product a try!