Beginning in early October, Microsoft will offer small business customers the opportunity to install Microsoft Security Essentials on up to 10 PCs in their network for FREE. For more details, check out the following announcement from the Microsoft Partner SMB Community Blog:

http://blogs.msdn.com/b/mssmallbiz/archive/2010/09/22/announcing-microsoft-security-essentials-available-free-to-small-businesses-in-october.aspx