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. Red Cross Antivirus, Peak Protection 2010, Pest Detector 4.1, Major Defense Kit, AntiSpySafeguard, ThinkPoint, and antiSpy Safeguard are exactly the same. The rogue that was selected will automatically start and perform a fake scan on your computer. While running, this program will also terminate many programs when you attempt to run them.
Source: https://www.bleepingcomputer.com/virus-removal/remove-fake-microsoft-security-essentials-alert