Microsoft Security Essentials has to be just about the worst anti-virus program there is. Every time a new version rolls out, MS oddly "updates" the previous versions to start detecting "viruses" more often, or to even give you a virus so that you upgrade. They did this with XP to Vista and 7.
That said, the executable is fine. Maybe redownload it. Something I'd recommend is Eset Smart Security. It's not free, but it's very good.