Microsoft announced it has <A HREF="http://news.zdnet.com/2110-1009_22-5755881.html?tag=zdnn.alert">closed its acquisition</A> of privately held security company Sybari Software. The East Northport, N.Y., software maker's products were designed to work with Microsoft Exchange and Lotus Notes messaging servers. Its Antigen file-filtering software promises to fight off viruses and spam on networks.