Microsoft Security Essentials (MSE) is a free antivirus software product created by Microsoft that provides protection against different types of malware such as computer virus, spyware, rootkits and trojan horses for Windows XP (x86), Windows Vista, and Windows 7 (both x86 and x64). Microsoft Security Essentials replaces Windows Live OneCare, a commercial subscription-based antivirus service and the free Windows Defender, which only protected users from adware and spyware. Unlike the Microsoft Forefront family of enterprise-oriented security products, Microsoft Security Essentials is geared for consumer use.

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... Microsoft Security Essentials is geared for consumer use.

Not any more!

According to the Microsoft Partner SMB Community blog:-

Now, some of you may ask whether Microsoft Forefront Client Security is a better security solution for your business. Well, that depends… If you want virus and malware protection to be centrally administered and integrated with group policy settings, then Forefront is a better option for you. But if you are comfortable with more of a set it and forget it solution, and are looking for an easy way to save money, Microsoft Security Essentials is definitely worth a look.

From last October, MSE is:-

... available to small businesses on up to 10 PCs for FREE

.