I recently started using Microsoft Security Essentials and I have version 2.1.1116.0. I keep it updated through the interface. SE reports that it's up-to-date. But the Windows Update website (I'm using Windows XP SP3) is offering an optional update. It's, Definition Update for Microsoft Security Essentials KB2310138 (Definition 1.117.1438.0). It's 71.3 MB. Does anyone know what this is or if I need it? It looks to me like the latest full installer of the definition database, but I really haven't a clue. I checked the "Details" link but it turned out to be useless. I also did some searching online but couldn't find an answer. Thanks.