Fake Anti-Virus Disguised as Microsoft Security Essentials

I found a Trojan fake alert virus today on a client’s PC that was masking itself as Microsoft security essentials, I was unable to distinguish bettween the trojan program and the real thing even though I have the AV on my home PC and am very familiar with it. The client never downloaded the real Microsoft security essentials AV and sure enough, once the virus was removed, there was no trace of essentials.