Microsoft Security Essentials - Recent Engine update performed
All MSE and Forefront users should ensure they are up-to-date for virus signature files. On September 16, 2010, Microsoft included an engine update which will enhance detection capabilities to better meet constantly changing security requirements.
http://blogs.technet.com/b/enginenotifications/archive/2010/09/16/antimalware-engine-1-1-6201-0-was-released-to-customers-on-16-september-2010.aspx
QUOTE: Antimalware Engine 1.1.6201.0 is released to all MSE and FCS customers on 16 September 2010. Signature package 1.91.0.0 is the first that contains this engine.
Affected products: Microsoft Security Essentials (MSE), Forefront Client Security (FCS)
Engine Version will be in the range of 1.1.620X.0