Microsoft Security Essentials is Most Used AV Protection in North America
"Microsoft notes that the free security software has been downloaded 30 million times so far."
http://www.dailytech.com/Microsoft+Security+Essentials+is+Most+Used+AV+Protection+in+North+America/article21879.htm
There is something in their numbers that makes me uncomfortable to believe them...
They oscillate very very much. The number of checked computers are small compared to the total market.
But, after all, I did not find any other market share information... avast team has just said, one time, that the numbers could be biased because, in one time, avast would have more computers in US than Symantec and they acknowledge that is not true.