Microsoft Corp. today released four patch bundles to fix at least eight security vulnerabilities in PCs powered by its Windows operating system and other software. The fixes are available through Microsoft Update or via Automatic Updates. Half of the flaws fixed in February’s patch batch earned Microsoft’s most urgent “critical” rating, meaning attackers could wield them to break into vulnerable systems with little or no assistance from users, aside from maybe convincing users to visit a booby-trapped Web site or open a specially-crafted e-mail. Two of the critical vulnerabilities reside in Microsoft’s Internet Explorer 7 Web browser (oddly enough, Microsoft says IE6 is not affected). The other two critical flaws Redmond fixed are found in Microsoft Exchange, an e-mail server program used by tens of millions of organizations. Andrew Storms, director of security operations for nCircle, a network security company, said the Exchange vulnerability is especially serious for businesses, because