With much press attention, Microsoft released its free Antivirus Engine called Microsoft Security Essentials. We had a quick look at it and while Microsoft has done a pretty good job altogether (quick, nice user interface, fairly decent signature database), it is what it is: an Antivirus Engine that is based on blacklists / heuristics.
However this means MSE won’t protect you against the sophisticated Trojans that we hear in the press almost daily. We have successfully infected a machine with enabled and up-to-date MSE with a new mutation of the Zeus Trojan that is active in the wild. (for the interested reader, here is a screencapture movie that also shows how TrustDefender protects you from Zeus).
So in our opinion MSE will not make any impact on the malware landscape at all, however it will most certainly take market share from the other Antivirus Vendors and put the pressure on them from a pricing point of view.