There are at least 36 AV software tools for the Windows Platform out of which about a dozen are free. Now, Microsoft Security Essentials joins that list. It is a free download that requires the OS where it is installed to be genuine Windows XP, Vista or Windows 7
It is lightweight & provides real-time protection against viruses, spyware, and other malicious software without hogging system resources as most AV software do.
MSE replaces Windows Live OneCare and Windows Defender.