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Yesterday was a big "Patch Tuesday" with 17 new security patches released by Microsoft, 9 of which were listed as "critical". The second Tuesday of each month is known as "Patch Tuesday", and this is when Microsoft regularly releases free security upgrades. As vulnerabilities are discovered, upgrades are made to the software to prevent hackers from exploiting the vulnerabilities. Full details on the April 12, 2011 Microsoft Security Bulletin can be found on the Microsoft web site.