I recently moved to Microsoft Security Essentials from Avira for my personal choice for Free based Anti-Virus software. The main issue I had with Avira was the random pop-up's I would get random times that would interfere with some important activities, mainly my Lord of the Rings Online time......during those pop-ups, my game would see a noticable slow down, so I thought it was time to move on from Avira. Other than that small issue, Avira has proved to be good Free based Anti-Virus choice in the past.
It's been about 1 week so far with Microsoft Security Essentials, and it certainly looks I will be keeping this install and letting it continue to run in the background. No unwanted pop-up's or any other type of interferece besides the required memory resources needed to run in the background. So far so good with this one from Microsoft. Definetly a great addition to the line up of available Free Anti-Virus software for Windows based systems.