Most every day here someone comes along and asks about a recommendation for an antivirus software and many SevenForums members use and recommend MSE. In addition to those recommendations being an indication of the popularity of MSE, note that MSE has once again become the target of a rogue.
As described at Bleeping Computer:
The fake Microsoft Security Essentials Alert
is a Trojan that attempts to trick you into thinking you are infected so that you will then install and purchase one of 5 rogue anti-virus
programs that it is distributing. When the Trojan is run it will masquerade as an alert from the legitimate Windows Microsoft Security Essentials Program anti-virus program.
Complete description and removal instructions are available at Remove the Fake Microsoft Security Essentials Alert Trojan